Is borrowing a pen to be returned as two pens, or a garment as two garments, considered usury?

The rule concerning loans is that the borrower must return an equivalent item for fungible goods and the value for non-fungible goods. It is not permissible to stipulate an increase in a loan, because a loan with an increase is usury (riba), based on the Prophet's (peace be upon him) saying: "Whoever gives or takes an increase has engaged in usury; the receiver and the giver are equal in this."

Therefore, if someone lends an unused pen, the borrower must return a similar pen, because manufactured pens nowadays are considered fungible goods.

However, if the pen was used, it becomes a non-fungible item, and its value on the day the borrower received it becomes a debt owed by the borrower. When repaying, there is no harm in paying an amount equivalent to that value, with the mutual consent of both parties.