Khums and Zakat Both are obligatory. But conditions for Zakat is totally different from Khums.
So let’s talk about Khums only.
Khums is on the surplus to annual expenses. If your income exceeds the annual expenses of yourself and your family, Khums (20%) should be paid from the excess. Therefore you cannot pay Khums in advance on that amount which has not in your hands and you are not sure whether it will remain or not.

As for Zakat, it is payable on cattle, crops and silver, raisins and dates in accordance to the Islamic rulings.