Stuart Stone
My understanding is that you offer a mortgage as security in exchange for a loan (you then assume the role of a borrower, customer etc)...you repay the loan over time (if it says in the loan that it's assignable & you agree to it, then you agree to fulfilling the liability to the lender on the understanding/agreement that they will pass it on to the assignee).
Once the loan agreement has been paid off by you, you want the original security (mortgage that you offered) back...it has an agreed upon value the same as the loan amount, so that's the amount you invoice them for, fault, default & lien them for.