An assumption agreement is a kind of legal document that you'll find commonly used in finance and real estate transactions. Basically, the agreement shifts the financial responsibility of the loan to a different borrower.Consider the following scenario. In real estate transactions, an assumption agreement allows a third party to "assume" or take over the loan of the property's seller. It allows one party to "assume" the rights and responsibilities of the other party. Documentary stamp taxes are payable on a mortgage assumption agreement only when the original mortgagor is released. An assignment and assumption agreement is a contract that allows one of the parties to transfer their contractual rights and duties to another party. An assumption clause in a mortgage contract allows a home's seller to pass responsibility for the existing mortgage to the new homebuyer.