Does the annual sum paid by my friend count as a halāl return on my unrecovered money, and is zakat obligatory only on this return or on the principal amount?

For the Mudarabah (profit-sharing) contract to be valid, it is a condition that a known, undivided percentage of the profit be specified for both the worker and the capital owner, and that the Mudarib (worker) does not guarantee the capital. If the Mudarabah is invalid (fasidah), there is a difference of opinion among scholars regarding how the profit should be divided. Some hold that all the profit belongs to the capital owner, and the worker receives wages for his labor. Others believe that if the Mudarabah was deemed valid, the worker should be given his share of the work. If the Mudarabah is valid, the Zakat on the capital is obligatory upon the owner, and the Zakat on the profit follows suit, with all of it being zakatable when a full year has passed. It is obligatory to make up for the Zakat of past years and to repent for any negligence in learning the rulings of Sharia. If equipment is invested in and not for sale, then what is subject to Zakat is its yield after a full year has passed.