Makdish a Sedei Achuza

I just saw an interesting Chasam Sofer regarding being makdish a sedei achuzah. The gemara in Bava Metziah (67B) has a discussion on various types of collateral (mashkon) that a borrower gives a lender. One type is called "nechyasah". The borrower gives the lender a field as a mashkon where the lender eats the fruits during the years of the loan and the value of the fruit is deducted from the amount that the borrower owes. Another type is called "asrah d'misalki" which means that the borrower has a right t o repay the loan and take back the mashkon anytime before the loan is due.

The Nimukei Yosef quotes the Rashba that there is no problem of ribis by asra d'misalki b'nechyasah. His rayah is from sedei achuzah. By a sedei Achuzah, a person is makdish the field to hekdesh and he can redeem it anytime before yoveil. The amount he pays depends on how many years there are left to yoveil. Teh question is, how is sedei achuza a rayah-there is no loan taking place.

The Chasam Sofer explains that when you are makdish the field you are not makdish the field but rather you are makdish the money-50 shekalim. However, instead of giving the money to hekdesh, you owe them the money and the field is a mashkon. So in reality you borrowed 50 shekalim from Hekdesh and gave them a field as a mashkon. Since you can pay back the loan i.e. redeem the field anytime, the maskon is an asrah d'misalki. Furthermore, since the amount of the loan is reduced based on how many years hekdesh held it, it is also nechyasah. Since the Torah says it works we see there is no ribis by asra d'misalki which is nechyasah