I deposited some USDT into my Card wallet (I have the physical plastic card) but the balance is grayed out. Apparently it cannot be spent, nor transferred back to my spot wallet.
Depositing and spending EURO instead works fine.
I guess that it's a bug and you should have not been allowed to transfer the funds there in the first place.
The card can spend only a handful of coins and for those it is possible to transfer in and out.
All in all, I suggest you
ask their support help you out and transfer your Tether out of the card wallet and back to the spot wallet. You may have to wait for that, but imho it's you who has tried things you shouldn't.
When that is solved and you have your Tether back in spot wallet, exchange them for BUSD which you'll be able to spend just fine with your card.