Thank you for reporting this issue. We have received your feedback regarding the ‘Simple trade’ acting unexpectedly within the trading terminal, especially with the USDT/FDUSD pair on Binance Spot. We understand the confusion you experienced with the buy button labeling and the fluctuating ‘Total’ field. We’ll investigate this behavior and will update you shortly.
You can see little lock. The one which is not locked will be changed with every ticker price change. Total is quote asset, amount is base asset. So if you want to lock total (use all 500 USDT for the order), you should type 500 in total field
It depends on the pair fee, base size step etc, its possible that dust will be left after order.