This is not how Belgian income tax works. Your pay gets divided up into tax brackets, and you pay a certain tax rate per tax bracket. You will pay a low rate for the first ~8.5k you earn, a higher rate for the next ~5k, higher still for the next ~10k and so on.
The result is that while you can be taxed more on additional income if it bumps your total income up to a certain tax bracket, you will only ever pay more money on the part that's actually above the lower limit for that bracket. This will never ever cause you to pay more taxes on the money you've already earned.
The result is that while you can be taxed more on additional income if it bumps your total income up to a certain tax bracket, you will only ever pay more money on the part that's actually above the lower limit for that bracket. This will never ever cause you to pay more taxes on the money you've already earned.