When a real estate purchase agreement or contract is executed and the terms are accepted by both parties, it is a complete and binding document. Should it become necessary to change any aspect of the previously agreed-to terms, an amendment is prepared. It is a document that makes a change to the ratified contract.
Real estate agents need to be sure to not confuse this with an Addendum.
A buyer and seller agree on the purchase of a piece of property and the contract is signed by all parties. Later, the closing date needs to be moved out to allow for a delayed survey. This would require that the closing date in the original agreement be amended.