How to fill out the Mortgage Assumption Agreement Document? Get to grips with assumption agreements.This simple guide explains their purpose, key elements, and how they affect your business dealings. Thinking of taking on a mortgage from a family member? Then you'll likely look into a mortgage assumption agreement. THIS MORTGAGE ASSUMPTION AGREEMENT (hereinafter referred to as the. "Agreement") made and entered into as of this ______(1)______ day of ______(2).