This mortgage assumption agreement allows a new borrower ("Borrower") to assume an existing mortgage from the original borrower. 1. Read the agreement carefully to understand the obligations.A mortgage assumption is the process of a buyer taking over, or assuming, the seller's existing home mortgage. A loan assumption is a lending process where a borrower formally takes over or "assumes" a current home loan from a home seller. Get to grips with assumption agreements. This simple guide explains their purpose, key elements, and how they affect your business dealings.