<PAGE>   1
                                                             EXHIBIT 10 (NNN)

                   MUTUAL ESTOPPEL AND MODIFICATION AGREEMENT

          THIS MUTUAL ESTOPPEL AGREEMENT is made as of this 15th day of June,
1998 by and among PNC BANK, NATIONAL ASSOCIATION (successor by merger to
Provident National Bank) (the "Bank"), THE PRUDENTIAL INSURANCE COMPANY OF
AMERICA ("Prudential") and EQK REALTY INVESTORS I ("EQK").

                                    RECITALS

          WHEREAS, the Bank, Prudential and EQK entered into a certain
Subordination and Intercreditor Agreement dated as of December 16, 1992 (the
"Intercreditor Agreement"); and

         WHEREAS, the parties wish to make certain statements and agreements
regarding the Intercreditor Agreement and the loans governed thereunder;

          NOW, THEREFORE, for good and valuable consideration, the adequacy and
receipt of which are hereby acknowledged, the parties hereto, intending to be
legally bound, agree as follows:

         1.       All capitalized terms not defined herein shall have the
meanings given such terms in the Intercreditor Agreement.

         2.       The Bank hereby certifies as follows:

                  (a)      The outstanding principal amount of the New Provident
Note, as amended and restated by the Fourth Amended and Restated Note (the "PNC
Restated Note") dated as of June 15, 1998 (collectively, the "PNC Note") is
$1,581,769;

                  (b)      The rate of interest under the PNC Note is either 
2-5/8% above the Euro-Rare (as such term is defined in the First Amendment to
Second Amended and Restated


<PAGE>   2


Loan Agreement dated as of December 15, 1995 (the "PNC First Amendment")
between EQK and the Bank) or 1-1/8% above the Prime Rate (as such term is
defined in the PNC First Amendment);

                  (c) There are no scheduled monthly payments of principal
required under the PNC Note; and

                  (d) To the best of PNC's knowledge, there exist no defaults
under the PNC Note and the Subordinate Loan Documents, nor do any circumstances
exist with which the passage of time, or the giving of notice, or both, would
constitute a default under the PNC Note or the other Subordinate Loan
Documents.

         3.       Prudential hereby certifies as follows:

                  (a)      The outstanding principal amount under the New 
Prudential Note, as amended by the Note, Mortgage and Loan Modification
Agreement dated as of December 15, 1995 and further amended by a First Amended
Note, Mortgage and Loan Modification Agreement dated as of December 15, 1996
and further amended by a Second Amended Note, Mortgage and Loan Modification
Agreement dated as of December 15, 1997 (the "Prudential Modification
Agreement") between Prudential and EQK (collectively, the "Prudential Note") is
$43,794,149.14;

                  (b)      The rate of interest under the Prudential Note is 
8.88% per annum;


                                       2
<PAGE>   3


                  (c)      The monthly payment of interest required to be paid
under the Prudential Note is $324,076.70; and

                  (d)      To the best of Prudential's knowledge, there exist no
defaults under the Prudential Note and the other Senior Loan Documents, nor do
any circumstances exist which with the passage of time, or the giving of
notice, or both, would constitute a default under the Prudential Note or the
other Senior Loan Documents.

         4.       Borrower hereby certifies that the statements made by the 
Bank and Prudential in paragraphs 2 and 3 above, respectively are true and
correct and that there exist no defaults under any of the Subordinate Loan
Documents or Senior Loan Documents nor do any circumstances exist which with
the passage of time, or the giving of notice, or both, would constitute a
default under any of the Subordinate Loan Documents or the Senior Loan
Documents.

         5.       Bank confirms to and agrees with Prudential and Borrower as
follows:  (a) that the Subordinate Loan Documents, including the amendments and
modifications hereinabove and that certain Third Amendment to Second Amended
and Restated Loan Agreement dated as of June 15, 1998 (the "PNC Third 
Amendment"), remain subordinate to the Senior Loan Documents, including the 
amendments and modifications thereto referred to hereinabove; (b) calculation
of the Contract Amount referred to in Section 4(b)(I)(A) of the
Intercreditor Agreement shall be based on the interest rate set forth in the
PNC Note; (e) the Prudential Pay Rate referred to in Section 4(b)(i)(B) of the
Intercreditor Agreement shall be the interest rate set forth in the Prudential
Note (e.g. 8.88% per annum); and (d) notwithstanding the fact that the Contract
Amount may be less than the Cap Amount, EQK is permitted and EQK hereby agrees
to make debt service payments to the Bank in an amount equal to, but not
exceeding, the Cap Amount.


                                       3
<PAGE>   4


         6.       The parties hereby agree and confirms (a) that all references
to any or all of the Senior Loan Documents or Subordinate Loan Documents
contained in the Intercreditor Agreement shall mean the Senior Loan Documents
as amended by the Prudential Modification Agreement and the Subordinate Loan
Documents as amended by the PNC Third Amendment and the PNC Restated Note; (b)
all of the terms and conditions of the Interceptor Agreement shall be extended
until all sums under the Senior Loan Documents have been paid in full; and (c)
that all of the terms and conditions of the Intercreditor Agreement remain in
full force and effect and binding upon the parties hereto and their respective
successors and assigns.

         7.       This Agreement may be executed in any number of counterparts 
which, taken together, shall constitute one and the same instrument.

         8.       This Agreement shall be governed by and construed in 
accordance with the laws of the Commonwealth of Pennsylvania.


                                       4
<PAGE>   5


         IN WITNESS WHEREOF, the parties have executed this Mutual Estoppel 
Agreement as of this 15th day of June, 1998.

                                          PNC BANK, NATIONAL ASSOCIATION


                                          By:/s/ Andrew D. Coler
                                             ----------------------------
                                          Title: Vice President
                                                 ------------------------

                                          THE PRUDENTIAL INSURANCE COMPANY
                                          OF AMERICA

                                          By: /s/ Paul D. Egan
                                              -----------------------------
                                          Title: V.P.
                                                ---------------------------

                                          EQK REALTY INVESTORS I

                                          By: /s/ Don Henry
                                             -------------------------------
                                          Title: V.P.
                                                 ---------------------------


                                       5