What should he do with the money he has from selling a phone he bought from a foreign website, after recovering the full purchase price from the seller, knowing that he used a personal discount voucher when buying it?

In summary: The phone's inability to work on all networks is considered a defect that permits rescission of the contract or taking compensation (arsh). In case of rescission, the phone must be returned. If it is damaged or sold, then a similar one must be returned, or its value on the day the similar item was lost. You must strive to reach the seller and return a phone like his, or its market value (at least $925). The discounts or other expenses you paid are irrelevant. If the seller cannot be reached after investigation, then its value must be given in charity on his behalf.