The sale of a property was subject to the buyer's being able to assume an...

A due-on-sale clause allows the lender to call the loan due when the property is sold or transferred, preventing the buyer from simply assuming the seller’s existing loan without lender approval. That directly matches the question’s wording: the buyer wanted to assume the existing loan, but the note contained a clause prohibiting that assumption. An assumption clause would allow or describe assumption, not prohibit it. A prepayment clause concerns paying the loan before maturity. A subordination clause changes lien priority between interests; it has nothing to do with whether a buyer may assume the loan. This belongs under Real Estate Transactions because the Pennsylvania title exam tests notes, mortgages, loan closings, recording, and settlement procedures.