Among the mistakes of those who fast: dealing in riba (usury or interest).

The Shaykh : And among these is that some people fast from food and drink, but, may Allah protect us, they deal in riba (usury or interest). An example of riba, though it has many forms, is when a person gives someone cash in exchange for more of it later, such as giving a thousand for a thousand and one hundred over the course of a year. This applies whether he does it explicitly or through a ruse, because some people do this explicitly, giving cash for more cash on credit, while others do it through a ruse. An example of the ruse is: he comes to a person and sells him a commodity in a nominal sale, saying, for example, "Take these bags of rice or cardamom or whatever else; these bags are worth, say, one hundred thousand." He sells them to him for one hundred and twenty thousand. Then, after the sale, the borrower sells them to the shopkeeper who originally bought them from the first person, and the borrower leaves with cash. We know with certainty that the owner of the cash he borrowed did not want this commodity, and we also know that the one who sold it to him and originally bought it from the shopkeeper did not want it either. If the shopkeeper had had any other type of money, he would have bought that, because the intention is to obtain cash for more cash, but this commodity is placed in between them.