This type of agreement transfers the obligations of one party under a contract 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.A mortgage assumption is the process of a buyer taking over, or assuming, the seller's existing home mortgage. A mortgage assumption agreement is basically you coming in and saying sign this paperwork I'm still responsible for the mortgage. An Assumption Agreement is a legal document through which one party transfers its obligations or duties under a contract to another party. In real estate transactions, an assumption agreement allows a third party to "assume" or take over the loan of the property's seller. Meanings assigned to those terms in the Purchase and Assumption Agreement. (a).