Despite the somewhat jaundiced view that law students get from studying the law of contract - that all contracts end by one party or the other breaking the contract - most contracts come to an end when both parties perform their part of the contract eg when one delivers the goods and the other pays for them.
In order for a contract to be discharged by performance the performance of the contract must be precise and exact. If this was not the case there would be a breach of contract. For example, if there is a contract between Max and Christine for the sale of his red pen to her for £5 and he delivers a black pen then the performance of the contract would not be precise and exact and therefore Max would have broken the contract.