Is zakat due on the full amount of 300,000, or on 100,000 after deducting a debt of 200,000, knowing that the debt will be paid within a month from a salary that is paid in installments over a year?

If you have money in excess of your needs, do not deduct the debt from the wealth on which zakat is due, but pay zakat on all of it. However, if you do not have money in excess of your needs, it is permissible to deduct the amount of the debt from the wealth on which zakat is due, and you only pay zakat on the remainder if it does not fall below the nisab.