A man took a loan from me and mortgaged his building with me. Then, I collected (and took) the building's rentals but he returned the (loaned) sum in full. Are the collected rents considered usurious interests, since the sum, in this way, is considered an interest-yielding debt?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

You have no right to take the yield of the mortgaged estate; you should give it to its owner once you collect it. You also have no right to withhold it for the mortgage is based on the estate itself. This goes back to the statement of the Prophet, sallallaahu ‘alayhi wa salam: "In no way would the object of mortgage be taken (from the mortgager in return for the debt for which it is mortgaged); the profit is his due, and the loss is born by him.” [Ibn Maajah and Maalik] [As-Suyooti: Hasan]

Any increase of the property belongs to the profit, and this is why it should be his (the mortgager's) own right.

Furthermore, since the mortgage never deprives the mortgager of the ownership of the object of mortgage, it is incumbent upon you to return the collected rents to the owner of the mortgaged building, and ask him to excuse you for the period for which you withheld it from him, for you have no right in it at all. There is consensus among scholars that it belongs to the mortgager.

Allaah Knows best.