Is it permissible to donate the sum of money that was to be paid to a friend in exchange for him bringing something, when that item has already been acquired from another source, and there is uncertainty about whether the friend actually obtained it, and difficulty in communicating with him to hand over the money?

Your transaction with him is a mandate from you to him to acquire the merchandise. If he brings it, you must pay him its price. If he does not bring it, you do not have to pay anything. If he informs you that he has acquired it, you may agree with him to sell it or take possession of it, or request cancellation of the sale from the seller. If this is done and he receives his right, then you owe nothing. Otherwise, the merchandise is yours, and you must pay him its price. It is not permissible to give charity on his behalf as long as it is possible to deliver it to him.