No. If you have a signed deal, the agreement to pay the buyer's closing costs is not a contingency and your offer is not subject to being replaced if a better one comes along. A contingency is something like; contingent upon selling or closing of the buyers property, contingent upon approval of an inspection, contingent upon obtaining financing. These issues are subject to something else happening that is clearly described and easily verified.
Closing costs should have a specified dollar amount, as discount points and other items can be validly described as closing costs and jack up the cost. Typically I would indicate, Seller agrees to pay $5000 (or whatever amount your lender and Realtor suggested) toward the buyer's allowable closing costs. If the contract were to state, "Unless a better offer comes along' then it could be a contingency, but otherwise I don't think so.
If you have any doubts about your contract, and the agent isn't clear, please have it reviewed by an attorney. We aren't attorneys and haven't reviewed your specific contract.