How is the zakat calculated on money given to a relative for investment, on the condition that the giver receives a share of the profit?

If the money paid is a loan, it is not permissible to take a percentage of the profit, for every loan that draws a benefit is usury (riba). However, if it is for participation in trade (mudarabah (definition) - profit-sharing), then it is permissible to take a share of the profit according to the agreement. The difference between the two is that the capital guarantee in a loan is on the borrower, while in mudarabah, it is on the capital owner. Zakat (definition), in the case of a loan, is obligatory only on the capital and is subject to the rulings of zakat on debt. As for mudarabah, it is obligatory on the capital and the share of profit.