Is the borrower sinful if he defrauds his friend to take from him the difference in the exchange rate of the dollar and gold, after he lent him an amount in dollars to be repaid in Egyptian pounds, and took another amount from him in Egyptian pounds to purchase goods that were not purchased, and the exchange rates and gold prices changed significantly?

The contract between you, stipulating that you give him dollars for him to deliver pounds after a period of time, is forbidden and invalid. Therefore, your companion must return the dollars to you, and you must return the pounds to him. If you both wish, you can complete a new exchange contract where both عوضين (countervalues) are taken possession of simultaneously. As for the pounds he took to buy goods for you but did not, the obligation is for him to return only what he took from you, without any increase, and it is not permissible for you to resort to stratagems to take anything from his money.