If a currency exchange merchant pledges to a buyer to purchase currency from him at an increased price beyond the original price, should its value fall within a week, does this condition invalidate the sale, and is it permissible for the buyer to complete the purchase and disregard this void promise, while avoiding selling it back to the original merchant?

The condition that obliges the seller to buy the commodity if the buyer does not find a seller for it is a void condition and has no اعتبار (legal weight). However, if the first seller wishes to buy the commodity from the one to whom he sold it, without any trickery involving usury (riba), then there is no harm in it. He is not obliged to buy it. Annulment of this sale is more preferable to avoid the dispute regarding its validity.