Final answer:
The legal device obligating repayment of a mortgage loan is typically the mortgage agreement itself. Banks may sell mortgage loans in the secondary loan market, and factors such as down payments, cosigners, and collateral help secure the loan's repayment.
Step-by-step explanation:
The legal device that obligates a borrower to pay back a mortgage loan is typically the mortgage itself, which is a loan secured by the collateral of specified real estate property that the borrower is obliged to pay back with a predetermined set of payments. When a family takes out a 30-year mortgage loan to purchase a house, they have a legal obligation to make payments to the lender over the duration of the loan. Banks consider these loans assets as they represent a stream of future payments. Banks may also sell these loans to other financial institutions in the secondary loan market, transferring the rights to receive the borrower's payments.
To measure the present value of a mortgage that is being paid over 30 years, banks or investors would estimate what another party in the market is willing to pay for it now, taking into account various factors such as interest rates, the borrower's creditworthiness, and the risk of default. Additional aspects that can secure a loan include requiring a down payment, cosigners, or collateral which provides further assurance that the loan will be repaid.