Is paying the mechanic the full amount for repairing a car – in exchange for repairing a part he does not currently own, but promises to buy and fix later, along with repairing another part that is available – considered usury (riba)? And what should be done if it is?

Expediting payment or the price in a sale does not constitute usury (riba). However, usury of delay (riba al-nasī'ah) occurs when the exchange of one of the two countervalues is delayed in the sale of usurious commodities, such as gold for gold or gold for silver. In such cases, possession must be taken in the contracting session, based on the Prophet's (peace be upon him) saying: "Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, and salt for salt—like for like, equal for equal, hand to hand. If these commodities differ, then sell as you wish, provided it is hand to hand."