Question On Khums
It is Ushr, not Khums. It is on the salary which is taxable. The taxes deducted from the salary and other taxes are counted as zakat paid to the government. After these deductions, if the amount of zakat is left to be paid, then you can pay the rest amount of zakat. and if the taxes exceed the amount of zakat you were supposed to pay, then you may count as a Nafal Sadaqa or can count if a portion of zakat is paid in advance.