<PAGE>   1
                                                                   Exhibit 10.20


                                         Real Estate Purchase Contract
                                     Industrial - Investment - Commercial
                                   Adopted by the Columbus Board of REALTORS

                                      It is recommended that all parties
                                be represented by legal counsel and a Realtor.

                                                              December 18 , 1997

1.   PROPERTY DESCRIPTION: the undersigned Buyer offers to purchase from the
     Seller through Broker(s), the following described real estate including,
     without limitation, all improvements, fixtures, appurtenant rights
     privileges, and easements located in the County of Franklin , the State of
     Ohio known as: 5501 Frantz Road, Dublin, Ohio 43017 -- a 5.766 acre site
     improved with a three-story office building of approximately 39,500 square
     feet.

2.   PRICE AND TERMS: The purchase price is See Addendum attached hereto for
     purchase price, Buyer's contingencies and additional terms. Dollars
     ($________) payable as follows: _____________________________________.

3.   CONTINGINCIES:
     (A)  Environmental Inspection: (This paragraph 3 (a) not applicable in
          number of days not inserted.) Within ________ days after the
          acceptance hereof, Seller agrees to permit the Buyer, Buyer's lender
          and the qualified, professional environmental consultant of either of
          them to enter the premises to conduct, at the expense of the Buyer, an
          environmental site assessment. Buyer agrees to indemnify and hold
          Seller harmless from any injury or damage caused by such inspection.
          If such assessment if obtained and the consultant recommends further
          inspection to determine the extent of suspected contamination or
          recommends remedial action, the Buyer, at Buyer's option, may notify
          the Seller in writing, within the above specified period, that the
          contract is null and void.

     (B)  Property Inspection: (This paragraph 3 (b) not applicable if number of
          days not inserted.) Buyer, at Buyer's expense, shall have ________
          days after the acceptance hereof to have the property and all
          improvements, fixtures and equipment inspected. Seller shall cooperate
          in making the property reasonably available for such inspection(s).
          Buyer agrees to indemnify and hold Seller harmless from any injury or
          damage caused by such inspection(s). If Buyer is not, in good faith,
          satisfied with the condition of the property as disclosed by such
          inspection(s), Buyer may terminate this contract by delivering written
          notice of such termination to Seller, along with a written copy of
          such inspection report(s), within the time period specified above,
          such notice and report(s) shall specify the unsatisfactory conditions.
          Failure of Buyer to so deliver written notice and copy of inspection
          report(s) within such time period shall constitute a waiver of Buyer's
          right to terminate pursuant to this provision.

     (C)  Other contingincies: See Addendum attached hereto for purchase price,
          Buyer's contingencies and additional terms.

4.   POSSESSION: Possession shall be given, subject to tenants' rights as
     tenants, upon closing.

5.   RENTALS AND OTHER PRORATIONS AND SECURITY DEPOSITS: SEE ADDENDUM ATTACHED
     HERETO.

6.   FIXTURES AND EQUIPMENT: The consideration shall include all fixtures owned
     by Seller including, but not limited to: built-in appliances, heating,
     ventilating, air conditioning (HVAC) and humidifying equipment and their
     control apparatus; stationary tubs; pumps; water softening equipment; roof
     antennae; attached wall-to-wall carpeting and attached floor coverings,
     curtain rods and window coverings including draperies and curtains;
     attached mirrors; light, bathroom and lavatory fixtures; storm and screen
     doors and windows, awnings, blinds and window air conditioners, whether now
     in or on the premises or in storage; garage door openers and controls;
     attached fireplace equipment; security systems and controls; smoke alarms;
     satellite TV reception system and components; all exterior plants and
     trees; and the following: (None if left blank)
     _______________________________________.

7.   DAMAGE OR DESTRUCTION OF PROPERTY: Risk of physical loss to the real estate
     and improvements shall be borne by Seller until closing, provided that if
     any property covered by this contract shall be substantially damaged or
     destroyed before this transaction is closed, Buyer may (a) proceed with the
     transaction and be entitled to all insurance money, if any, payable to
     Seller under all policies covering the property, or (b) rescind the
     contract and thereby release all parties from liability hereunder by giving
     written notice to Seller and Broker within ten (10) days after Buyer has
     written notice of such damage or destruction. Failure by Buyer to so notify
     Seller and Broker shall constitute an election to proceed with the
     transaction.

8.   CONDITION OF IMPROVEMENTS: Seller agrees that upon delivery of deed, the
     improvements constituting part of the real estate shall be in the same
     condition as they are on the date of this offer, reasonable wear and tear
     expected.

9.   EVIDENCE OF TITLE: Seller shall furnish and pay for an owner's title
     insurance commitment and policy [ALTA Form B (1992 REV. 10-17-92)] in the
     amount of the purchase price. The title evidence shall be certified to
     within thirty (30) days prior to closing with endorsement not before 8 a.m.
     on the business day prior to the date of closing, all in accordance with
     the standards of the Columbus Bar Association, and shall show in Seller
     marketable title in fee simple free and clear of all liens and encumbrances
     except: (a) those created by or assumed by Buyer; (b) those specifically
     set forth in this contract; (c) zoning ordinances; (d) legal highway; and
     (e) covenants, restrictions, conditions and easements of record that do not
     unreasonably interfere with present lawful use. Buyer shall pay any
     additional costs incurred in connection with mortgage title insurance
     issued for the protection of Buyer's lender. If Buyer desires a survey,
     Buyer shall pay the cost thereof. If title to all or part of real estate in
     unmarketable, as determined by Ohio law with reference to the Ohio State
     Bar Association's Standards of Title Examination, or is subject to liens,
     encumbrances, easements, conditions, restrictions or encroachments other
     than those excepted in this contract, Seller shall, within thirty (30) days
     after a written notice thereof, remedy or remove any such defect, lien,
     encumbrance, easement, condition, restriction or encroachment or obtain
     title insurance without exception thereof. In the event Seller is unable to
     remedy to insure against the defect within the thirty (30) day period, the
     Buyer may declare this contract null and void. At closing, Seller shall
     sign an affidavit with respect to off-record title matters in accordance
     with the community custom.

10.  CONVEYANCE AND CLOSING: At closing, Seller shall pay transfer taxes and
     deed preparation and shall convey, at closing, marketable title (as
     described in paragraph 9) to the real estate by deed of general warranty
     (or appropriate fiduciary deed if Seller is a fiduciary) in fee simple,
     with release of dower, if any. The date of closing shall be: December 29,
     1997.

11.  TAXES AND ASSESSMENTS: With regard to further assessments, Seller warrants
     that, as of the acceptance hereof, no improvements or services to the site
     or area have been installed or furnished that would result in the costs
     being assessed against the real estate, and no written notification has
     been received by Seller from public authority or owner's association of
     future improvements that would result in costs being assessed against the
     real estate. Real estate taxes and assessments are subject to retroactive
     change by governmental authority. The real estate taxes for the property
     for the current tax year may changes as a result of the transfer or as a
     result of a change in the tax rate. SEE ADDENDUM ATTACHED HERETO.

12.  BUYER'S EXAMINATION: BUYER IS RELYING SOLEY UPON HIS OWN EXAMINATION OF THE
     REAL ESTATE AND INSPECTIONS HEREIN REQUIRED, IF ANY, FOR ITS PHYSICAL
     CONDITION, CHARACTER, AND SUITABILITY FOR BUYER'S INTENDED USE AND IS NOT
     RELYING UPON ANY REPRESENTATIONS BY THE BROKER(S), EXCEPT FOR THOSE MADE BY
     BROKER(S) DIRECTLY TO THE BUYER IN WRITING.
<PAGE>   2
13.  INDEMNITY: Seller agrees to defend, indemnify and hold harmless Broker(s),
     and their agents and employees for any cost or liability that may be
     incurred by or imposed on Broker(s) for any breach by Seller of any
     representation of warranty or for any misrepresentation or concealment of
     fact by Seller in connection with the property.

14.  ENVIRONMENTAL DISCLAIMER BY BROKER: Buyer and Seller acknowledge that
     Broker(s) have made no independent investigation to determine whether
     hazardous materials exist in, on or about the property. Buyer and Seller
     understand that any such determination requires the expertise of a
     specialist in hazardous materials, the retaining of which is the
     responsibility of Buyer and/or Seller and not that of the Broker.

15.  DEPOSIT: Buyer has deposited with the Broker the sum receipted for below,
     which shall be returned to Buyer, upon Buyer's request, if no contract
     shall have been entered into. Upon acceptance of this contract by both
     parties, Broker shall deposit such amount in its non-interest-bearing trust
     account to be disbursed, subject to collection by Broker's depository, as
     follows: (a) deposit shall be applied on purchase price or returned to
     Buyer when transaction is closed; (b) if Seller fails or refuses to
     perform, or any contingency is not satisfied or waived, the deposit shall
     be returned; (c) if Buyer fails or refuses to perform, this deposit shall
     be paid to Seller. If the parties are unable to agree upon the disposition
     of the deposit, then upon the request of either Buyer or Seller for the
     return or payment of the deposit, the Broker holding the deposit shall give
     written notice to the other party of such request, and shall advise the
     other party that such deposit shall be returned or paid in accordance with
     such request unless the other party delivers written objection thereto
     within 20 days after receipt of such notice. If the Broker does not receive
     any written objection within such 20-day period, then the Broker shall
     return or pay such deposit in accordance with such request. If the other
     party objects in writing within such 20-day period, Broker shall retain the
     deposit until (i) Buyer and Seller have settled the dispute; (ii)
     disposition has been ordered by a final court order; or (iii) Broker
     deposits said amount with a court pursuant to applicable court procedures.
     The return or payment of such deposit shall not in any way prejudice the
     rights of Seller, Buyer or Broker(s) in any action for damages or specific
     performance.

16.  MISCELLANEOUS: This contract constitutes the entire agreement and no oral
     or implied agreement exists. Any amendments to this contract shall be in
     writing, signed by Buyer(s) and Seller(s) and copies provided to them. This
     contract shall be binding upon the parties, their heirs, administrators,
     executors, successors and assigns. If this contract involves seller
     financing, it may not be assigned. Time is of the essence of all provisions
     of this contract. All provisions of this contract shall survive the
     closing. In compliance with fair housing laws, no party shall in any manner
     discriminate against any Buyer or Buyers because of race, color, religion,
     sex, familial status, handicap or national origin. Paragraph captions are
     for identification only and are not part of this contract.

17.  EXPIRATION AND ACCEPTANCE: This offer shall remain open for acceptance
     until 6:00 p.m. Columbus, Ohio time on December 19, 1997, and a signed copy
     shall be returned to all parties upon acceptance.

18.  BROKER'S FEE: Seller shall pay a brokerage fee of $0.00 of the purchase
     price in connection with this transaction, payable at closing. Seller and
     Broker acknowledge that there are no other Broker(s) involved in this
     transaction except as follows:



Buyer hereby makes the foregoing offer this 18th day of December 1997.
BORROR REALTY COMPANY


By: */S/ Randolph B. Robert
    --------------------------------          --------------------------------
    Buyer                                     Buyer

Address:  5501 Frantz Road                    Phone
          Dublin, Ohio 43017                       ---------------------------


Deed to:  Borror Realty Company


--------------------------------------------------------------------------------
Name of Buyer's Attorney



Seller agrees to and accepts the foregoing offer this 18th day of December 1997.

DOMINON HOMES, INC.


By: */S/ Robert A. Meyer, Jr.
    --------------------------------          --------------------------------
    Seller                                    Seller

Address:  5501 Frantz Road                    Phone
          Dublin, Ohio 43017                       ---------------------------


--------------------------------------------------------------------------------
Name of Seller's Attorney


NOTE: AGENCY DISCLOSURE STATEMENT: Buyer and Seller acknowledge having reviewed
and signed the attached Agency Disclosure Statement as required by Ohio law.


ALL PARTIES TO THIS CONTRACT MUST BE PROVIDED WITH A COPY.

Broker acknowledges receipt of the sum $0.00 by cash/check, which shall be held,
deposited and disbursed pursuant to paragraph 15 above.

By:
   ----------------------------------------------------------------------------
<PAGE>   3
                           ADDENDUM TO SALE CONTRACT
                 DOMINION HOMES, INC. CORPORATE OFFICE BUILDING


This Addendum is made to the sale contract between Borror Realty Company
("Buyer") and Dominion Homes, Inc. ("Seller"). Buyer and Seller agree as
follows:


     1. Price and Terms. The purchase price is Three Million Nine Hundred Fifty
Thousand Dollars ($3,950,000.00), less the current principal amount of the
existing first mortgage to State Farm Insurance Company, which Buyer will
assume, and less a closing credit of Two Hundred Thousand Dollars ($200,000.00)
for repairs to the roof and sidewalks, and for expansion of the parking lot to
be performed by and paid for by Buyer. If the actual cost of repairs and parking
lot expansion is less than the $200,000.00 credit, the difference shall be
returned to Seller. The referenced repairs and actual cost of repairs and
parking lot expansion and the $200,000.00 credit shall be returned to the Seller
by not later than May 1, 1998. The net purchase price shall be paid in cash at
closing.


     2. Rentals and Other Prorations and Security Deposit. Existing rents,
future rents from current tenants other than Seller, and security deposits shall
be retained by Seller. Accordingly, there will be no prorations, including other
expenses between the parties at closing.


     3. Taxes and Assessments. Seller as future tenant shall be solely
responsible for payment of all recurring property taxes, special taxes, and
assessments against the property. Accordingly, there shall be no proration of
taxes and assessments between the parties at closing.



BORROR REALTY COMPANY                          DOMINION HOMES, INC.

By: */s/ RANDOLPH B. ROBERT                    By: */s/ ROBERT A. MEYER, JR.
    ----------------------------                   ----------------------------
    Randolph B. Robert                             Robert A. Meyer, Jr.
    Vice President                                 Senior Vice President