A Muslim lawyer promises his Muslim client that he will not charge him any fees.

Answer

If the lawyer had agreed and promised not to charge fees, then it was not permissible for him to demand any fees from the client. He is answerable to Allah Ta'ala for violating his promise. But when the client paid the account without even raising objections, he implied his agreement. He voluntarily paid the money which has now become the property of the lawyer. He may not now demand a refund. If the lawyer had indeed made the promise, then morally he should refund the money. But there is consensus of entire mankind – Muslim and non-Muslim – that the moral domain does not exist in the realm where lawyers operate. It is therefore futile to pursue the moral angle. Legally the lawyer does not have to refund the money since it was given to him by the client.