The concept of part performance is one based on an equitable principle called detriment.

Many legal concepts that are accepted in certain areas, were born out of equitable concepts that existed before there were written laws. Part performance is one such concept.

The idea behind it is that if one party to a contract did something that puts him in a worse position, either financially or otherwise, then it assumes that they acted on the belief that a contract existed and they did that action to further that contract.

So for example, if someone were to pay the deposit for a property, even if the contract was oral and not in writing, the principle is that the person who paid the deposit was willing to part with their money because they believed a contract was in place—they acted to their detriment. This act would be seen as part performance, and constitute the enforcement of the contract.

Even if the contract was in writing, and one party was of the impression that it was subsequently cancelled or unenforceable, if the other party performed part or completed their obligation under said contract, then equity requires that the contract be enforced, to correct the detriment to that party.

In law, this principle is enforced using the legal principle of specific performance. This is usually the remedy that is sought by the party that acted to their detriment when trying to enforce the contract in court.

This is the remedy that is aimed at ensuring the other side performs their side of the contract, doing “specifically” what they would have done if they had performed what they were obligated to do—hence the name. This remedy aims to put the parties, as far as possible, in the position they would have been in had the contract been performed according to its terms.

This allows for the party who acted to their detriment to be on level terms with the other party, that is, shared detriment or exchange, such as money for land. The part performance is proof of an arrangement for a contract and the remedy sought as a result is specific performance.

It is important that if one party wishes to rescind the contract, they do so before one party acts on the contract, so as to avoid being forced to either provide compensation for their loss or to perform the contract in full.