What should one do if he swore to give his friend one thousand dinars if he was the caller, and then it became clear that he was indeed the caller, and he does not possess the amount nor can he give it due to debt and unemployment?

If you take an oath and are unable to fulfill it, wait until you are able to do so. Expiation (kaffarah) is not required of you until you despair of fulfilling what you swore to do, because an oath not limited by a specific time does not become a breach (hanth) until death. If you do not fulfill what you swore to do, then expiation for the oath becomes binding on you.