From: Rabbi Moshe Sofer
To: Rabbi Yechezkel Segal
A father's will contained conflicting clauses: one tied the annual 26-gold-Müntz Torah study payment to investment returns on 400 gold coins, while the other imposed it as an unconditional lien on the son's house. The Chasam Sofer rules that the 400 coins functioned as a purchase price for the son's acceptance of a perpetual encumbrance on the house, reconciling both clauses and obligating the full 26 Müntz annually regardless of investment returns. Because the arrangement is a sale of a property interest rather than a loan, there is no concern of ribis and no heter iska is required.
Source: Shu"t Chasam Sofer CM §15