If the lender stipulates a condition that he will take back more than he gives – for example, he gives 10 kilograms of wheat and stipulates that he will take back 11 kilograms, or he gives ten eggs and stipulates that he will take back eleven eggs – this is usury (ribā) and unlawful (ḥarām).
In fact, if it is agreed that the lender will perform a task for him or will return the loan along with some other commodity – for example, he stipulates that the £10 he has given on loan must be returned along with one matchstick – this is also usury and unlawful.
Furthermore, if he stipulates a condition that the item being taken on loan must be returned in a particular manner – for example, he gives an amount of gold that has not been crafted and stipulates that gold that has been crafted [such as a piece of jewellery] must be returned – again, this is usury and unlawful. However, if the borrower himself returns the loaned item with an extra amount without such a thing being stipulated, there is no problem; in fact, it is recommended.