A condition that MUST be met before the sale contract is enforceable is known as a(n):

An amendment refers to a formal change or addition made to an existing contract. While amendments can modify contract terms, they do not serve as conditions that must be satisfied for the contract to be enforceable. Instead, they alter the contract's provisions after it has been created.

A rider is an additional provision or document that is attached to a contract to modify its terms or conditions. Similar to amendments, riders do not represent prerequisites that must be fulfilled for a contract to be enforceable. They are typically used to expand or clarify the existing agreement.

A restriction is a limitation placed on the actions of the parties involved in the contract, which might relate to how the property can be used or transferred. However, restrictions do not constitute conditions necessary for the enforceability of the sale contract itself; they are more about regulating behavior post-agreement.