We do not charge the trading fee upfront, meaning the fee amount is deducted after the trade is executed.
So if the balance amount is less than the trading fee amount then your fund's balance will go into negative as per the fee charged in the last trade.
For example, you have 1000 USDT and want to buy BTC using the full USDT balance. Then you will receive the complete BTC amount and the fee amount will get deducted from the USDT balance after the trade execution. As you had used the entire USDT balance to buy BTC, the fee amount will go into negative which will get settled in your future sell orders in USDT markets or P2P buy orders.