I'm pretty sure I've seen a thread asking the same question a while back and that the verdict was no because it could give gil an IRL value (e.g. you usually sell commissions for $30, but in game you sell them for 3mil or something like that), but I could be mistaken. Maybe someone else will drop by with a more definitive answer. (But if it is allowed, I'd totally love to do FF commissions for gil, too. xD )