Is it permissible or impermissible to sell products to someone who is in debt at a price higher than the market price, then repurchase them from him at a price lower than the selling price, without the buyer having taken possession of the product?

The aforementioned transaction is impermissible because it is a forbidden form of 'inah (buy-back) transaction, and its reality is an usurious loan. The commodity was introduced to circumvent usury. Thus, the contract is outwardly a sale but inwardly usury. This is what Ibn al-Qayyim clarified in his statement: "If someone wants to sell one hundred for one hundred and twenty deferred, and then gives a commodity for the deferred price, then buys it back for the immediate price, while neither of them has any interest in the commodity whatsoever; rather, it is as the jurist of the Ummah (meaning Ibn 'Abbas) said: 'Dirhams for dirhams, with a silk cloth (or intermediary item) entering between them.'"