He should appeal his ban, once he is unbanned he is supposed to log on the server and give you the 300 million. If you're not able to come on an Admin+ should hold on to the 300 million till you can come on. Failure for him to do so will result in a permanent ban.
What I would recommend you guys do, speak about it somewhere, come up with a deal, propose it to whoever is handling the appeal or both paste it on the thread to make us aware. If he's not paid you back on the agreed terms by the deadline, he can be banned again.