This sort of thing used to happen quite regularly when we were in Premiership for games against the likes of Man United and Newcastle United et al.
Knowing they could sell out to their home fans they’d allocate us say 3,000 tickets and then you’d have the option of another 1,500 after that but if you don’t sell out you have to pay for all those unsold tickets.
iIt sounds like Grimsby were unwilling to pay for any unsold tickets at first and so told their fans if they didn’t sell them all they’d receive a refund (making it look like Wednesday were the bad guys) but now it looks like they’ve agreed to pay for any unsold tickets for which they’ll probably receive about 40% back anyway when gate receipts are shared anyway.