Capital Gain on Sale of Property where sale consideration was directly paid to bank against due loan

A client sold his property, part commercial and part residential, to pay off long due loan for which the property was mortgaged. all the sale consideration received by the client was directly paid by the buyer to the bank to pay off the loan and make the property lien free. the client is left with no money from the sale. he has no other property on his name. he has no other source to pay the capital gain tax liability.

what can he do?