What should a person do who bought clothes from a store in Syria, but was unable to pay the seller there, and is it permissible to donate the amount to the poor with the intention of repayment?

It is not enough to donate the right (or due) if it is possible to deliver it. Rather, one must wait for the owner of the shop to return, or send the right with a trustworthy person, or deposit it into his account. If this is not possible and one despairs of reaching him, then the money should be given as charity on his behalf. The owner of the right, upon his return, will then have the choice between approving the charity or taking his right.