If you have signed an agreement and the buyer has not honored his end of the bargin and the 90 days stipulated are over can you cancel

You have not given enough information for me to provide a meaningful response.  By "agreement" I will assume you mean a contract.  A contract ordinarily ties both parties to perform some act (or perhaps to refrain from doing something he/she otherwise has a right to do).  Ordinarily, when one of the parties fails to perform as required by the contract, the other is released from his/her duty to perform.  Sometimes, when one of the parties to a contract fails to perform, the other party is damaged in some way.  When that happens, the damaged party may have a cause of action against the defaulting party -- that is, he may be entitled to sue to recover his/her losses.  If there are no damages, then it would seem a default by one of the parties would entitle the other to declare the contract null and void.