Under what condition can a modification of a contract be valid at common law?

A modification of a contract can be valid at common law if it requires consideration for the new modification. At common law, a contract modification is not enforceable unless both parties provide something of value in return for the new terms. This is a fundamental principle based on the notion that agreements should have mutual exchange; hence, each party must give or receive something to make the modification legally binding.

While some jurisdictions may allow modifications to be made in writing or even without consideration in certain situations (like in the case of a written contract that specifically allows for modifications), the general rule under common law is that consideration is necessary to ensure that the modification is supported by a valid legal foundation. Additionally, while it is important for all contracts to be fair and considerate of both parties, the key requirement for validity here pertains to the presence of consideration rather than fairness or equity.