The form below is a very basic one. Chances are you will need something more comprehensive.
AGREEMENT BY SELLER NOT TO LEASE PROPERTY
Seller, _____________________________________, warrants that the property at _______________________________________________ is not available for lease extending beyond ______________ (date). Furthermore, Seller agrees that seller will not take on any lease of the premises that extends beyond _______________ (date), unless first obtaining written approval of _______________________________, Buyer.
These forms are provided AS IS. They may not be any good. Even if they are good in one jurisdiction, they may not work in another. And the facts of your situation may make these forms inappropriate for you. They are for informational purposes only, and you should consult an attorney before using them.