פירוש על קידושין 37:15

Rava derives another halakhah from the ruling of R. Yose son of R. Yehudah. When the father sells her to the master, it is like a loan. The master gives money and the daughter owes the master a certain amount of work. She herself is the collateral, held by the master until she completes the work. And then when he betroths her, he forgives her at least a perutah’s worth of work. So he is essentially forgiving a loan and returning the collateral. So too a man can betroth a woman through a loan she owes him and by returning to her the collateral that he holds.