Dear Jasneet, for all regular employees, "salary" is paid monthly. The definition of salary is money that is paid regularly for doing work. Therefore, for all regular employees, whether they work in the sales department or otherwise, you must have a fixed component of the salary. You cannot link the entire salary itself to the sales target. While you may obtain an "acceptance" letter from the employees for the condition that you wanted to impose on them, nevertheless, putting up such a condition itself is wrong.
In view of this, I recommend you modify your views about the payment of monthly wages and salaries to the employees. You may bifurcate gross salary into fixed and variable components. The fixed salary cannot be less than the minimum wages as prescribed by the state government.
Thanks,
Dinesh Divekar