It is permissible for a person to lend a portion to his friend, [on condition] that he should use it to make profit for the creditor, and grow it until it reaches two portions. The money belongs to the creditor until that time. Once it becomes worth two portions, the two portions will go to the creditor, and any other profit will go to the debtor – so long as the creditor pays for the debtors services until that point. [Translation by Rabbi Steve Greenberg]