What is the ruling on buying gift cards for less than their face value and then using them to purchase goods at their full value? Is this considered usury (riba)? If so, what must the buyer do? Should they donate the difference in the initial purchase price or the difference in the selling price after purchasing the goods?

One must differentiate between purchasing the card through lawful or unlawful means:

First: If you purchased a stolen card with the knowledge that it was stolen, you have committed a sin. You must repent to Allah and return the stolen item or its value to its owner. If that is not possible, then donate its value to the poor and needy. You are also entitled to demand from the thief what you paid him.

Second: Purchasing a card that allows for obtaining unknown gifts is religiously prohibited due to ignorance (jahalah) and gharar (uncertainty), as the sold item is not clearly identified. This is the view of the majority of jurists, while some scholars permitted it under conditions not present here. The sale of "something from unknown things" is impermissible. If the invalidity of the sale is established, it must be rescinded by returning the card and the money to their respective owners.

Third: This transaction is not usurious (ribawi), because the card is not money exchanged for money; rather, it is a commodity (an offer) exchanged for money.