That's not quite how it works. When the bond company writes the bond, they don't just take the $2m from the client and then the client is off the hook. They ask the client to put up collateral. So in order for Trump to secure this bond, he would have had to set aside the full amount of the bond and say "If the verdict doesn't get reduced on appeal, I am giving you all this stuff, and not using it for anything else in the meantime".

All Trump is getting for that fee is not actually having to sell those assets now, and have any overage refunded if the appeal gets the verdict reduced. (Given the interest rates right now, the passive income on that kind of cash is not trivial).