Assumption of a Mortgage is a unique approach to acquiring property, considered assumption property buying. What happens in this type of transaction, the buyer takes liability, and thus the payments for an existing mortgage. 

When the seller has a property that has a mortgage secured against it, the buyer has the choice of assuming the existing mortgage or taking a new one. The mortgage lender still has to approve the new buyer, and can deny the assumption of the mortgage. Some mortgages include in the underwriting that the mortgage is or is not assumable. The seller has to ensure they get a release from the loan, this way they are not liable at all for any defaults or payment on the loan. 

One good benefit for the buyer to assume the mortgage is that the existing mortgage rates can be lower than market rates at the time of purchase. You also save a little bit on closing costs since the underwriting process is simpler than drafting a whole new loan. 

It is important to note that most sellers would rather sell their property for a lump sum, to reap benefits of property increases. And lenders may want that the new buyer creates a new mortgage with updated terms. 

On another note the equity that the seller made on the property will have to be compensated for, usually in the form of a higher down payment. Understanding the financials and cost savings are important to see if this deal structure is right for you. 

This assumption of mortgage is a rare occurrence but for parties that may work together or know each other and came to this agreement; this can be a great way to build a portfolio.