<PAGE>   1
                                                                Exhibit 10(bb)



                        FUND AMERICAN ENTERPRISES, INC.
                          The 1820 House, Main Street
                          Norwich, Vermont 05055-0850


                               December 15, 1995


Mr. Michael C.  Allemang
Senior Vice President and Chief Financial Officer
Source One Mortgage Services Corporation
27555 Farmington Road
Farmington Hills,  Michigan   48334

                            STOCK PURCHASE AGREEMENT

Dear Mike:

     This Stock Purchase Agreement sets forth the agreement of Fund American
Enterprises, Inc. (the "Purchaser") to purchase shares of Common Stock
("Stock") of Source One Mortgage Services Corporation ("SOMSC") on the terms
and conditions stated herein.

     1.  Stock Purchase. The Purchaser will, at any time or from time to time,
purchase Stock aggregating up to $100,000,000 within 5 business days after
receipt of written notice from SOMSC during the term hereof, in the amount (in
whole shares) so specified by SOMSC, at a purchase price per share of Stock
equal to the Per Share Price.  The purchase price of Stock is payable by (at
the Purchaser's option) (i) cash or (ii) the transfer to SOMSC of certain
specified Assets (defined below) having a fair market value equal to the
aggregate purchase price or (iii) any combination of (i) and (ii).  At each
closing, SOMSC will deliver certificates for the Stock purchased at such
closing registered in the name of the Purchaser or its nominee.  At any closing
involving the transfer by the Purchaser to SOMSC of securities in payment of
all or any portion of the purchase price of Stock, certificates evidencing such
securities shall be delivered to SOMSC duly endorsed or accompanied by duly
completed stock/bond powers, or by any other method of transferring ownership
that is mutually agreeable to the parties.

     2.  "Specified Assets" means, initially, the investment securities listed
on the New York Stock Exchange, described in Schedule A (the "Initial
Securities").  Thereafter,

(i)  the Specified Assets shall also include any proceeds received under
     paragraph 10 from the sale of the Initial Securities or any other
     Specified Assets and any Substitution Securities received under paragraph
     11 for the Initial Securities or any other Specified Assets, and any
     distributions or dividends payable in respect of Specified Assets and any
     security resulting from a conversion, modification, reclassification or
     other similar transaction with respect to any Specified Assets, and

(ii) the Specified Assets shall not include any securities or cash which are
     transferred to SOMSC pursuant hereto, or disposed of in compliance with
     this paragraph 2, or any Stock or distribution of any kind with respect
     thereto, or any securities which have been surrendered by the Purchaser 
     in any conversion, modification, reclassification or other similar
     transaction as stated in (i) above or any securities which are sold
     pursuant to paragraph 10 or deliverable in exchange for Substitution
     Securities received under paragraph 11.

<PAGE>   2


The Purchaser agrees that it will not transfer or subject to a lien any of
Purchaser's assets or properties which constitute Specified Assets so long as
this Agreement remains in effect, except that (i) the Purchaser may transfer
Specified Assets to SOMSC at a closing held hereunder and (ii) the Purchaser
may dispose, as the Purchaser sees fit in its sole discretion, of Specified
Assets so long as the value of remaining cash and cash equivalents, together
with the fair market value of the remaining Specified Assets, is at least $100
million.

Nothing in this Agreement shall restrict the Purchaser in any way from
disposing, as the Purchaser sees fit in its sole discretion, of some or all of
assets or properties of the Purchaser which do not constitute Specified Assets.
Also, it is understood and agreed that the Purchaser remains the owner of the
Specified Assets for all purposes, retaining all voting and other rights,
unless and until some or all thereof are transferred to SOMSC at a closing held
pursuant to this Agreement.

     3.  The fair market value of securities for all purposes of this Agreement
shall be determined by the Purchaser applying the valuation principles stated
in Schedule B, using market information for the last market day prior to the
valuation day.  In the case of the Initial Securities, their agreed fair market
values as of December 11, 1995 is as set forth in Schedule A.  In the case of
any Substitution Securities, their initial fair market values will be the
amount at which they are credited for substitution purpose pursuant to
Paragraph 11.  In each case, the Purchaser from time to time may, and prior to
any transfer to SOMSC hereunder or any substitution therefor pursuant to
paragraph 11, will adjust the initial (or previously adjusted) fair market
value as may be necessary to reflect any events, developments or circumstances
occurring after the date hereof or of their transfer to the Purchaser which in
the Purchaser's reasonable judgment materially change their fair market value,
determined in accordance with the valuation principles stated in Schedule B.
Purchaser agrees that it will promptly take all appropriate action to maintain,
at all times during the effective period of this Agreement, the value of cash
and cash equivalents, together with the fair market value of Specified Assets,
determined in accordance with the valuation principles stated in Schedule B, in
an amount not less than $100,000,000.

     4.  "Per Share Price" means the price of Stock, as determined as of the
most recent available quarterly balance sheet date, pursuant to the terms of
the valuation formula set forth on Schedule C.

     5.  Term.  The term of this Agreement shall continue from the date hereof
until the earlier of (i) the Specified Assets are exhausted by transfer(s) to
SOMSC pursuant hereto and (ii) the Purchaser and SOMSC mutually agree in
writing to end the term of this Agreement (which mutual agreement will be
effective on and as of the date specified therein).

     6.  Mutual Representations and Warranties.  Each party hereby represents
and warrants to the other that on and as of the date hereof;

     (a)  Each is a duly organized and validly existing corporation in good
standing under the laws of its jurisdiction of incorporation and any other
jurisdiction in which it has its principal office.  Each has been duly
authorized by all necessary corporate action (including without limitation any
necessary vote of security holders) to enter into and carry out this Agreement.

     (b)  No consent, permit, notice, registration, waiver or other action by
any third party or any governmental agency is required to be obtained, made or
given by it in connection with entering into or carrying out this Agreement.
Neither the entering into nor carrying out of this Agreement will cause any
breach or violation of, or any lien against its properties or assets under, or
any acceleration of any of its obligations pursuant to, any agreement,
commitment, law, statute, regulation or order to which it or any of its
properties are subject.

<PAGE>   3


     (c)  It has not engaged any broker, finder or similar person in connection
with the transactions contemplated hereby.

     (d)  It is an "accredited investor" as that term is defined in SEC
Regulation D, and is acquiring any securities to be transferred or issued to it
pursuant hereto for its own account.

     (e)  It is familiar with the affairs of the other party and (in the case
of SOMSC) is familiar with the affairs of the issuers of the Initial
Securities.  Except for the express representations and warranties made herein,
it is relying totally on its own evaluation of the risks and benefits
associated with this Agreement and the securities which may be transferred or
issued to it hereunder.

     7.  Additional Representations, Warranties and Agreements of the Parties.

         (a)  The Purchaser represents and warrants to SOMSC THAT:

         (i)  It owns 100% of the Initial Securities, free and clear of
all liens,

             (ii)  It has conducted no business since its incorporation 
activities and holding investments.

            (iii)  It is not an "investment advisor" required to be registered 
under the Investment Advisors Act of 1940, as amended (the "Advisors Act").

The Purchaser agrees that if it should ever be required to be registered under
the Advisors Act, it will so register on a timely basis and will comply with
the requirements of the Advisors Act.

         (b)  SOMSC represents and warrants to the Purchaser that it is not an
"investment company" required to be registered under the Investment Company Act
of 1940, as amended (the "Company Act"), and agrees that if it should ever be
required to be registered under the Company Act, it will so register on a
timely basis and will comply with the requirements of the Company Act.

     8.  Resales.  Each party agrees that it will not resell any securities
transferred or issued to it hereunder except in a transaction registered under
the securities Act of 1933, as amended, or any successor statute or in a
transaction exempt from such registration, established to the reasonable
satisfaction of the transferor or issuer, as the case may be.

     9.  Covenants of the Purchaser.  Purchaser agrees that during the term of
     this Agreement it will not:

     (a)  Engage in any material business other than owning the Specified
Assets and other investment securities.

     (b)  Voluntarily incur any indebtedness or obligations which would or
might constitute a claim enforceable against the Specified Assets (other than
under this Agreement and under a credit facility extended by The Chase
Manhattan Bank, N.A. and other financial institutions to the Purchaser and
other borrowers in principal amount not to exceed $75,000,000).

     (c)  Subject any Specified Assets to any lien or permit any Specified
Assets to become subject to any lien,

     10.  Sale.  Notwithstanding anything in this Agreement, the Purchaser may,
at any time and from time to time, at its election sell some or all of the
Specified Assets to third parties not affiliated with the 



<PAGE>   4

Purchaser in arms'-length transactions for cash and/or securities which
thereupon become part of the Specified Assets.


     11.  Substitution.  Notwithstanding anything in this Agreement, the
Purchaser may, at any time and from time to time, at its election, exchange
cash and/or securities in the Specified Assets for cash and/or securities
("Substitution Securities") held by the Purchaser or any of its affiliates
thereof with an aggregate fair market value not less than the aggregate fair
market value of the cash and/or securities to be exchanged, all determined as
of the date of substitution, provided that the Purchaser, promptly after such
substitution, delivers to SOMSC its calculation of the fair market values of
all assets included in the  exchange, which calculation confirms its compliance
with this paragraph 11.

     12.  Separate Portfolio Accounting and Management.  If any investment
securities are transferred to SOMSC pursuant to this Agreement, SOMSC, as sole
and absolute owner of such securities, will retain Fund American Enterprises,
Inc. as portfolio manager with respect to such investment securities pursuant
to a mutually acceptable Investment Services Agreement.

     13.  Miscellaneous.

     (a)  This Agreement is governed by the internal law of New York State.
Exclusive jurisdiction and venue for its enforcement shall lie in the state and
federal courts of the State of New York.

     (b)  Each party will pay and bear its own expenses in connection with this
Agreement and the enforcement hereof, except that in any proceeding primarily
between the parties for the enforcement hereof, the prevailing party may
recover its reasonable attorney's fees.  Each party will pay and bear the cost
of any stock transfer tax applicable to the transfer or issuance of securities
by it.

     (c)  This Agreement is the entire agreement of the parties as to its
subject matter, and supersedes any other agreements between the parties
relating to the subject matter hereof.  Nothing in this Agreement may be waived
or amended except by a writing signed by the parties hereto.  Nothing in this
Agreement is intended to create any third-party beneficiary rights.

     If the foregoing correctly states our agreement, please so indicate by
signing an enclosed copy of this letter to me.


                               Very truly yours,
 

                               -----------------------------------
                               for Fund American Enterprises, Inc.

Agreed:



--------------------------------------------
for Source One Mortgage Services Corporation

Schedules
A.   Initial Securities
B.   Valuation Principles
C.   Valuation Formula

<PAGE>   5


                                                                      SCHEDULE A



                        FUND AMERICAN ENTERPRISES, INC.
                             SCHEDULE OF SECURITIES
                                  VALUED AS OF
                               DECEMBER 15, 1995




<TABLE>
<CAPTION>
                                                     MARKET VALUE
SECURITY                                     SHARES     PER SHARE         TOTAL
<S>                                       <C>        <C>           <C>
Zurich Reinsurance Centre Holdings, Inc.  1,900,000        $29.00   $55,100,000
San Juan Basin Royalty Trust              8,000,000          6.00    48,000,000
                                                                   ------------
                                                                   $103,100,000
                                                                   ============

</TABLE>

<PAGE>   6



                                                                      SCHEDULE B



                              VALUATION PRINCIPLES



The following valuation principles shall be applied when determining the value
of the Specified Assets for purposes of this Stock Purchase Agreement.

For any security which is publicly held or so listed or publicly traded, such
security shall be valued at its daily Closing Price.  "Closing Price" on any
day of any security means (1) if such security is listed on the New York Stock
Exchange, the last sale price, regular way, or, in case no such sale takes
place on such day, the average of the closing bid and asked prices, regular
way, or, (2) if such security is not listed or admitted to trading on the New
York Stock Exchange as reported in the principal consolidated transaction
reporting system with respect to securities listed on the principal national
securities exchange on which such security is listed or admitted to trading or,
(3) if such security is not listed or admitted to trading on any national
securities exchange, the last quoted sale price or, if not so quoted, the
average of the high bid and low asked prices in the over-the-counter market, as
reported by NASDAQ. The source of the information used to compute the Closing
Price of any such security shall be The Wall Street Journal.

For any security which is not publicly held or is not listed or publicly
traded, such security shall be valued at the transferee's carrying value of the
security, calculated in accordance with generally accepted accounting
principles, as of the date of transfer.

<PAGE>   7


                                                                      SCHEDULE C



                               VALUATION FORMULA

                    SOURCE ONE MORTGAGE SERVICES CORPORATION
                             VALUATION METHODOLOGY



-    The valuation per share equals the company valuation divided by a
     number equal to the positive difference between the number of all shares
     of capital stock issued and outstanding and the number of shares issued as
     part of any "Capital Infusion" (as hereinafter defined) by Fund American
     Enterprises, Inc. ("FAE"),  Fund American Enterprises Holdings, Inc.
     ("FAEH"), and/or any "Person" (as hereinafter defined).  The share
     valuation should be rounded to the nearest 1/100 of a cent.

-    The initial (6/30/86) total company valuation is the acquisition
     price;  $258,148,000.  This also equals the 6/30/86 GAAP book net worth
     under purchase accounting (except that it does not include capitalized
     acquisition costs).  Since the initial number of shares outstanding is
     2,600,000, the initial valuation per share is $99.2877.

-    Company valuations will be performed on a formula basis not less than
     quarterly for purposes of the plan and will be equal to:

     -     current GAAP book net worth minus the GAAP effect of any Capital
           Infusion,

     -     minus current GAAP book value of in place mortgage servicing
           portfolio,

     -     plus current "formula value" of in place mortgage servicing
           portfolio (purchased and originated),

     -     minus the current GAAP book value of capitalized acquisition costs,

     -     minus current GAAP book value of the initial purchase accounting
           write up related to good will, buildings, FNMA stock, in process
           mortgage originations ("pipeline"), systems development and the
           branch network,

     -     plus the 6/30/86 excess purchase price that would have resulted had
           the in place servicing portfolio been valued and capitalized at its
           formula value on that date, and excluding capitalized acquisition
           costs ($907,509).  (This amount is a constant for all periods and
           equals $109,765,000).

-    The "formula value" of the mortgage servicing portfolio is a
     percentage (rounded to the nearest 1/1000 of a percent) multiplied by the
     total outstanding principal balance of the portfolio.  This servicing
     value excludes the "pipeline" of mortgages in process but not yet closed.

-    This portfolio valuation percentage is determined initially as of
     6/30/86 and first updated as of 12/31/86. Thereafter, the percentage will
     be updated quarterly.  The percentage will remain constant between the 
     quarterly updates, unless the Board of Directors (the "Board") of Source
     One Mortgage Services Corporation (the "Company") and FAEH Chairman decide
     more frequent changes are needed.

<PAGE>   8


-    This percentage will be developed from the Company's "bid model,"
     using data for the actual in place servicing portfolio as of 6/30/86,
     12/31/86, 3/31/87, etc.  The model (which includes certain supplemental
     calculations such as insurance, income and tax benefits) produces
     after-tax cash flows which are discounted at an after-tax rate.  The
     discounted cash flows equal the estimated value of the servicing
     portfolio.  This value divided by the total in place principal balance
     equals the portfolio valuation percentage.

-    The model's after-tax cash flows are composed of servicing income,
     minus servicing expense, minus/plus taxes (all on a cash basis).  Some of
     the highlights of the model are as follows:

     -     Cash servicing income includes servicing fees, interest savings
           (due to escrow balances and cash flow "float"), plus ancillary
           income such as insurance commissions, late charges, assumption fees
           and prepayment penalties,

     -     Cash servicing expense includes 100% of all variable servicing
           costs and 85% of all fixed servicing costs.  This does not include
           any Company interest expense, principal payments on debt or
           amortization of capitalized purchased servicing costs.  The expenses
           are based on actual costs for the latest 12-month period, plus a
           reasonable inflation assumption for future years.

     -     Taxes are calculated at the expected effective rate for each year
           (federal, state and local on a stand-alone basis) times:  (cash
           servicing income, less cash servicing expense, less non-cash
           purchased servicing amortization).  This amortization is based on
           the actual tax basis of the purchased servicing rights (not on the
           total servicing valuation) and on the actual tax basis amortization
           schedule.

-    The discount rate used in the portfolio valuation is based on the
     average daily yield of 10-year U.S. Treasury Bonds (as determined from the
     Federal Reserve Board "Statistical Release"), rounded to the nearest 1/100
     of a percent, plus a 700 basis point risk premium.  The average yield is
     for the period beginning 10 business days before the "as of" date for the
     bid model and ending 10 working days after.  For example, the initial
     period would be the 21 business days from 6/16 to 7/15/86 with an average
     Treasury rate of 7.42%.  The after-tax discount rate = (average pre-tax
     Treasury rate + 700 basis points) X (1 - posted effective tax rate).

-    The prepayment rate will be a weighted average of the rates
     experienced over the 48 months immediately preceding the valuation date.
     The weighting distribution will be as follows:  50% for the most recent 12
     months, 30% for the prior 12 months and 10% for each of the remaining 2
     prior 12-month periods.  Annual changes in the rate will be limited to a
     cap and floor of plus or minus 20%.  (Cumulative quarterly changes during
     a calendar year will be limited to 5%, 10% and 15%, respectively, of the
     prior year-end rate.)  The first prepayment rate will be calculated as of
     December 31, 1985.  Therefore, the prepayment rate on the initial
     valuation date (June 30, 1986) will be capped at a 10% increase from the
     weighted average rate on December 31, 1985.  A separate rate (rounded to
     the nearest 1/100 of a percent) will be determined for residential and
     commercial loans.


-    The interest saving on escrow balances for P & I float is based on the     
     average A1/P1 commercial paper rate for the latest 36 months, plus dealer
     cost, plus backup line cost, minus balance borrowing cost.  (The following
     pages provide additional details on various assumptions.)

-    All assumptions used in the bid model will be agreed to by the
     Company and FAEH management.  Further, the Company Board and the FAEH
     Chairman will agree on the treatment 


<PAGE>   9


     of any Capital Infusion with the intent that the effect of any Capital
     Infusion on the operation of this formula shall be neutralized to the
     fullest extent possible.


-    As used herein:  The term "Capital Infusion", means any infusion of
     new capital in the Company, after October 1, 1991, by FAE, FAEH and/or any
     Person for which FAE, FAEH and/or such Person receives capital stock of
     the Company.  (The term "Capital Infusion" includes, but is not limited
     to, the $300,000,000 capital infusion in November, 1991, (which capital
     infusion consisted of a portfolio of common equity securities and other
     investments then valued at approximately $300,000,000) by FAE for which
     FAE was issued 1,660,118 shares of Class A Common Stock (including certain
     treasury shares of Class A Common Stock) of the Company.)  The term
     "Person" means any person or entity other than (i) those persons listed on
     Annex I attached hereto and (ii) any employee stock ownership plan or
     similar plan of the Company.


<PAGE>   10


                                              SERVICING PORTFOLIO VALUATION



INPUT REQUIREMENT                             BASIS OF DETERMINATION

Initial Servicing                             Actual costs over the 12-month 
costs per loan                                period immediately preceding the 
                                              valuation date.  Servicing costs 
                                              will include full variable costs 
                                              and an 85% fixed cost allocation.

Servicing costs                               Estimate based on anticipated 
growth rate                                   inflation and assuming no 
                                              escalation of fixed costs due to 
                                              increased cost fixed cost 
                                              allocation.

Insurance income/                             Estimate of future income based 
Miscellaneous fee income                      on experience over the prior 3 
                                              years.

Prepayment Rate                               The prepayment rate will be a 
                                              weighted average of the rates
                                              experienced over the 48 months
                                              immediately preceding the
                                              valuation date.  The weighting
                                              distribution will be as follows: 
                                              50% for the most recent 12 months,
                                              30% for the prior 12 months and
                                              10% for each of the remaining 2
                                              prior 12-month periods. Annual
                                              changes in the rate will be
                                              limited to a cap and floor of plus
                                              or minus 20%.  (Cumulative
                                              quarterly changes during a
                                              calendar year will be limited to
                                              5%, 10% and 15%, respectively, of
                                              the prior year-end rate.)  The
                                              first prepayment rate will be
                                              calculated as of December 31,
                                              1985.  Therefore, the prepayment
                                              rate on the initial valuation date
                                              (June 30, 1986) will be capped at
                                              a 10% increase from the weighted
                                              average rate on December 31, 
                                              1985.  A separate rate (rounded 
                                              to the nearest 1/100 of a 
                                              percent) will be determined for 
                                              residential and commercial loans.

Foreclosures                                  The number of loans is based on 
                                              current FHA experience.  To be 
                                              reviewed and updated at 
                                              quarterly valuation dates.

Foreclosure loss per loan                     Estimate of Company foreclosure 
                                              losses on GNMA pools based on 
                                              loan loss experience.  To be 
                                              reviewed and updated at quarterly
                                              valuation dates.

Escrow balance per loan                       Based on the average escrow 
                                              balance available over the 
                                              previous 12 months.


<PAGE>   11


INPUT REQUIREMENT                             BASIS OF DETERMINATION

Interest earned on                            Average A1-P1 commercial paper 
escrow/P&I balances                           rate over the 36 months 
                                              immediately preceding the 
                                              valuation date, plus the cost of
                                              a back-up line of credit, plus 
                                              dealer costs, less the cost of 
                                              escrow available lines.  Rates 
                                              utilized in determining the 
                                              average will be AA rates as 
                                              published by the Federal Reserve.

Escrow interest cost                          Estimate based on 13 states 
                                              which require interest payments 
                                              on escrow funds held.  To be 
                                              reviewed and updated at quarterly
                                              valuation dates.

Escrow growth rate                            Estimate, to be held constant for
                                              future evaluation periods.

Principal and interest float                  Estimate float available on 
                                              FHA/VA and conventional loans.  No
                                              float assumed available on GNMA
                                              loans (float benefits are offset
                                              by interest advanced on   
                                              delinquent loans) or commercial
                                              loans.  To be reviewed and updated
                                              at quarterly valuation dates.

Reserve requirement                           Estimate based on current 
                                              experience and objectives.  To 
                                              be reviewed and updated at 
                                              quarterly valuation dates.

Tax rate                                      Federal taxes applied as 
                                              projected under current law.  
                                              State taxes assumed at 2%.

Discount rate                                 The discount rate will be applied
                                              to cash flows after adjustment for
                                              the effective tax rate in the
                                              period.  The discount rate used in
                                              the portfolio valuation is based
                                              on the average daily yield of
                                              10-year U.S. Treasury Bonds       
                                              (as determined from the Federal
                                              Reserve Board "Statistical
                                              Release"), rounded to the nearest
                                              1/100 of a percent, plus a 700
                                              basis point risk premium.  The
                                              average yield is for the period
                                              beginning 10 working days before
                                              the "as of" valuation date and
                                              ending 10 working days after.