<PAGE>   1

                                                                   EXHIBIT 10.20

Letter Agreement dated June 18, 2000 between Registrant and Martin J. Wygod


<PAGE>   2


                           HEALTHEON/WEBMD CORPORATION
                            3399 PEACHTREE STREET NE
                             400 THE LENOX BUILDING
                             ATLANTA, GEORGIA 30326

June 18, 2000
Mr. Martin J. Wygod
River Drive Center 2
669 River Drive
Elmwood Park, New Jersey

Re:      AGREEMENT AND PLAN OF MERGER (THE "AGREEMENT") DATED FEBRUARY 13, 2000
         BETWEEN HEALTHEON/WEBMD CORPORATION, A DELAWARE CORPORATION ("PARENT"),
         AND MEDICAL MANAGER CORPORATION, A DELAWARE CORPORATION, (THE
         "COMPANY"), AND AMENDMENT NO. 1 TO THE AGREEMENT, DATED AS OF JUNE 18,
         2000, BETWEEN PARENT AND THE COMPANY (TOGETHER, THE "AMENDED
         AGREEMENT"); AGREEMENT AND PLAN OF MERGER (THE "MERGER AGREEMENT")
         DATED FEBRUARY 13, 2000 BETWEEN PARENT, AVICENNA SYSTEMS CORPORATION, A
         MASSACHUSETTS CORPORATION ("ASC"), AND CAREINSITE, INC., A DELAWARE
         CORPORATION ("CAREINSITE" AND COLLECTIVELY WITH THE COMPANY AND EACH OF
         THEIR DIRECT AND INDIRECT SUBSIDIARIES, THE "COMPANIES"), AND AMENDMENT
         NO. 1 TO THE MERGER AGREEMENT, DATED AS OF JUNE 18, 2000, BETWEEN
         PARENT, ASC AND CAREINSITE (TOGETHER, THE "AMENDED MERGER AGREEMENT"
         AND COLLECTIVELY WITH THE AMENDED AGREEMENT, THE "AGREEMENTS").

Dear Mr. Wygod:

         Parent is in negotiations to acquire the Companies pursuant to the
above referenced Agreements which provide for the acquisition of the Company by
Parent pursuant to the merger of the Company with and into Parent and the
acquisition of CareInsite by Parent pursuant to the merger of CareInsite with
and into ASC (the "Mergers"). Your execution of this letter agreement is an
inducement to Parent entering into the Agreements providing for the Mergers as
well as in consideration for appointing you as Co-Chief Executive Officer of
Parent. In conjunction therewith, we request that you agree to become bound by
the noncompetition, nonsolicitation and confidentiality provisions set forth on
Exhibit A hereto effective upon the completion of the Mergers.

         As you know, the shares of Company common stock and CareInsite common
stock which you own (the "Current Shares") will each be converted into shares of
common stock of Parent at the Effective Time (as defined in the Agreements)
based on the Exchange Ratios as set forth in the Agreements and the shares of
Company common stock and CareInsite common stock which you have options to
purchase (the "Current Options") will each be converted into options to acquire
shares of the common stock of Parent at the Effective Time based on the Exchange
Ratios in the Agreements.


<PAGE>   3

         By agreeing to the terms of this letter, you confirm to us that (i) you
own 5,532,682 shares of the Company common stock, (ii) you own 39,000 shares of
CareInsite common stock, (iii) you own 136,000 Company options, (iv) you own
450,000 CareInsite options, (v) there are no written agreements pertaining to
your employment relationship with either the Company or CareInsite other than
option agreements pursuant to which you were issued the Current Options, (vi)
you will become bound by the noncompetition provisions described on Exhibit A
upon the consummation of the Mergers, and (vii) you are a resident of the State
of California.

         As additional consideration for your agreements set forth herein,
Parent agrees that after the Effective Time, in the event of the cessation of
your employment with Parent or the Co-Chief Executive Officer or Chief Executive
of Parent for any reason, each Current Option held by you shall continue to vest
in accordance with the applicable schedule, and shall otherwise be treated for
purposes of the terms and conditions thereof, as if you remained employed
through the earlier of (i) the first anniversary of the last scheduled vesting
date for such Current Option or (ii) the material breach by you of the
provisions of Exhibit A to this letter agreement.

         If you wish to accept our offer to serve as Co-Chief Executive Officer
of Parent following the Merger and the other terms of this letter, please
indicate by signing below and returning the signed copy to the undersigned at
the address set forth above. If the Mergers are not consummated, this letter and
Exhibit A hereto shall be null and void. We are excited about the future of our
combined companies and value your role in making our efforts successful.

         This letter and Exhibit A hereto constitute the entire agreement of the
parties and shall supersede any and all previous contracts, arrangements or
understandings between the parties relating to the subject matter hereof,
including without limitation, the letter agreement dated as of February 13, 2000
between you and Parent.

         Nothing contained in this letter shall be deemed to restrict any of
Parent, the Company, CareInsite or Martin J. Wygod from taking any action that
would otherwise be permitted pursuant to the terms of the Agreements and this
letter.

                                    Sincerely,

                                    Healtheon/WebMD Corporation


                                    By: /s/ Jack Dennison
                                       ----------------------------------------
                                       Authorized Representative

Accepted and Agreed:


/s/ Martin J. Wygod
--------------------------
Martin J. Wygod

June 18, 2000


                                       2
<PAGE>   4

                                                                       EXHIBIT A

                   Noncompetition; Nonsolicitation Provisions

1.       Background. Martin J. Wygod ("Wygod") acknowledges and agrees that:

                  (a)      Parent is currently negotiating to acquire the
Companies in the Mergers pursuant to the Agreements.

                  (b)      The Companies are currently engaged in the following
businesses: (i) development or provision of an Internet-based healthcare
electronic commerce network that links physicians, payers, suppliers or
patients, (ii) facilitating or processing administrative or clinical healthcare
transactions, (iii) clinical and administrative healthcare related electronic
commerce business, and (iv) development or provision of physician practice
management information systems or other healthcare software systems relating to
administrative and clinical functions, to physicians, practice associations,
management service organizations, physician practice management organizations or
other providers of healthcare services (collectively, the "Business"). The
Business is conducted by the Companies throughout the United States.

                  (c)      Wygod holds a substantial equity interest in the
Company and CareInsite in the form of common stock of both the Company and
CareInsite as well as options to acquire the common stock of both the Company
and CareInsite ("Target Equity"), which will be converted into common stock or
options to acquire common stock of Parent upon the consummation of the Mergers
("Parent Equity").

                  (d)      As a result of holding Target Equity, Wygod will
benefit from the consummation of the Mergers because, among other things, the
Exchange Ratios in the Agreements place significant value on Wygod's Target
Equity and Parent Equity to be received in exchange for Target Equity.

                  (e)      In order to protect the Business (defined above) of
the Company for which Parent is paying a substantial price, and as a condition
to Parent's execution of the Agreements and the appointment of Wygod as Co-Chief
Executive Officer, Parent is requiring Wygod to agree to become bound to the
provision of this Exhibit A upon the consummation of the Mergers.

                  (f)      The provisions of this Exhibit A are reasonable in
light of the substantial benefit that will accrue to Wygod through the exchange
in the Mergers of his Target Equity for Parent Equity and are reasonably
necessary to protect the Business that Parent is acquiring at a substantial
price.

2.       Noncompete. Effective as of the Effective Time and conditioned upon the
Merger being consummated and Wygod being appointed to the officer position of
Co-Chief Executive Officer (whether or not Wygod accepts such appointment),
Wygod agrees that for a period ending on the earlier of (i) the second (2nd)
anniversary of the later of the date (the "Termination Date") of


<PAGE>   5

Wygod's resignation or removal as Co-Chief Executive Officer, Chief Executive
Officer or as a director of Parent or any successor to its Business or (ii) the
third (3rd) anniversary of the Effective Time (the "Restrictive Period"),
without the prior written consent of Parent, Wygod shall not Compete (defined
below) with the Business acquired by Parent, except as otherwise permitted under
this Section 2. For purposes of this Agreement, "Compete" shall mean: (i) within
the Territory (defined below), to engage in a business or business activities
that are either (A) substantially similar to, or (B) competitive with, the
Business, in each case as engaged in by the Companies on the Effective Time
including changes in and expansions of such Business reasonably anticipated at
the Effective Time which are implemented or substantial steps are taken to
implement prior to the Termination Date (collectively, a "Competitive
Business"); (ii) to assist any person or entity (whether in a managerial,
financial, employment, advisory or other capacity or as a stockholder or owner,
except as set forth in clause (iii) below) to engage in a Competitive Business;
or (iii) to own any interest in or to organize a corporation, partnership or
other business or organization which engages in a Competitive Business;
provided, however, that nothing in clause (iii) above shall prohibit Wygod from
acquiring or holding, for investment purposes only, less than five percent (5%)
of the outstanding publicly traded securities of any corporation which may
compete directly or indirectly with the Business; or less than five percent (5%)
of the outstanding securities of any corporation, partnership or other business
or organization, whether or not publicly traded, which competes directly or
indirectly with the Business so long as he is not employed by and does not
consult with, or become a director of or otherwise engage in activities for such
competing company; provided further that this provision shall not apply in the
event the Companies or Parent or Parent's direct or indirect subsidiaries or any
person deriving title to the goodwill of the Business of the Companies being
acquired or shares of the Companies being acquired ceases to carry on a business
comparable to the Business (including anticipated changes in and expansions of
the Business which are implemented or substantial steps are taken to implement
prior to the Termination Date) within the Territory; provided further that this
provision shall not prevent or impair Wygod from performing usual investment
banking services for a person or entity engaged in a Competitive Business if
such services do not materially relate to or involve such Competitive Business.

                  "Territory" shall mean (a) the area within a 100 mile radius
of that office of the Company from which Wygod performed the majority of his
services during the one-year period ending on the earlier of his resignation or
removal as a director of Parent, or the 3rd anniversary of the Effective Time
(the "Applicable Date"), (b) the state in which Wygod is resident on the
Applicable Date, and (c) any other state in the United States in which the
Companies develop, distribute or provide their business services or products as
of the date of the Effective Time.

3.       Confidentiality. Wygod acknowledges that in the course of serving as
Co-Chief Executive Officer of Parent and as a result of his relationship with
the Companies prior to the Merger he has had and will continue to have access to
and will learn information that is proprietary to, or confidential to Parent and
the Companies and that concerns the Business including the operation,
methodology and plans of Parent and the Companies and their Affiliates (as
defined below), including without limitation, business strategy and plans,
financial information, trade secrets, market information developments (as
defined below), information regarding acquisition and other strategic partner
candidates and customer information (collectively, "Proprietary


                                       2
<PAGE>   6

Information"). Wygod agrees that during the period beginning on the Effective
Time and ending upon the later of (i) the end of the Restrictive Period or (ii)
the first anniversary of the Termination Date, he will keep such Proprietary
Information confidential and will not disclose directly or indirectly any such
Proprietary Information to any third party and will not misuse, misappropriate
or exploit such Proprietary Information in any way except as required by law or
regulatory body. Upon his resignation or removal as Co-Chief Executive Officer
or Chief Executive Officer of Parent, Wygod shall immediately return to Parent
all copies of Proprietary Information in his possession (except his Rolodex).

4.       Nonsolicitation. During the period beginning on the Effective Time and
ending upon the end of the Restrictive Period, Wygod shall not directly or
indirectly without the express written approval of the Board of Directors of
Parent, solicit any customer, or any person or entity who is reasonably expected
to become a customer of Parent, the Companies or any entity the equity of which
is owned at least 40% by Parent or a Company (an "Affiliate") for any commercial
pursuit which is a Competitive Business. During the period beginning on the
Effective Time and ending upon the end of the Restrictive Period, Wygod shall
not directly or indirectly solicit or induce, or attempt to induce, any
employees, agents, or consultants of Parent, the Companies or their Affiliates
to leave the employ of Parent, the Companies or their Affiliates or to do
anything from which Wygod is restricted by reason of this letter agreement, nor
shall Wygod, directly or indirectly, offer or aid others to offer employment to
or interfere or attempt to interfere with any employees, agents or consultants
of Parent, the Companies or their Affiliates.

5.       Construction. Wygod hereby expressly acknowledges and agrees as
follows:

                  (A)      the covenants set forth in Sections 2 through 4 above
are reasonable in all respects and are necessary to protect the legitimate
business and competitive interests of Parent in connection with Parent's
acquisition of the Business pursuant to the Agreement; and

                  (B)      it is the intention that the provisions of this
Exhibit A comply in all respects with California Business and Professional Code
ss. 16601. In the event that any provision of this Exhibit A shall be held
invalid or unenforceable by a court of competent jurisdiction by reason of the
geographic or business scope or the duration thereof of such covenant, or for
any other reason, such invalidity or unenforceability shall attach only to the
particular aspect of such provision found invalid or unenforceable as applied
and shall not affect or render invalid or unenforceable any other provision of
this Exhibit A and the provision shall be construed as if the geographic or
business scope or the duration of such provision or other basis on which such
provisions have been challenged had been more narrowly drafted so as not to be
invalid or unenforceable.

6.       Enforcement; Remedies. Wygod covenants, agrees and recognizes that
because the breach of the covenants, or any of them, contained in Sections 2
through 4 hereof may result in immediate and irreparable injury to Parent,
Parent shall be entitled to seek an injunction restraining Wygod from any
violation of Sections 2 through 4 to the fullest extent allowed by law. Wygod
further covenants and agrees that in the event of a material breach of any of
the respective covenants and agreements contained in Sections 2 through 4
hereof, the period during


                                       3
<PAGE>   7

which Wygod is obligated to refrain from competing shall be extended for the
entire period of such breach. Wygod further covenants, agrees and recognizes
that, notwithstanding anything to the contrary contained herein, in the event of
a material breach of any of the respective covenants and agreements contained in
Sections 2 through 4 hereof, which remains uncured 30 days after written notice
from Parent, further vesting with respect to the Current Options shall cease.
Parent's entitlement to seek injunctive relief or ceasing any further Current
Option vesting, as described in this Section 6, shall be Parent's sole and
exclusive remedy in the event that Wygod breaches any covenant or agreement
contained in this Exhibit A; provided, however, that in the case of any wilful
material breach by Wygod of the covenants and agreements contained in Sections 2
through 4 hereof, nothing herein shall be construed as prohibiting Parent from
pursuing any other legal or equitable remedies that may be available to it for
any such breach, including the recovery of damages from Wygod.


                                       4