So, you want to add a changeable increment field to all your employees. And only the best employee gets the increment. Correct me if I'm wrong.
1. Create a Salary head, add it to all your salary groups. Call it 'Allowance – employee of the month' or whatever you want.
2. If the value for the salary head is ZERO, it won't be displayed in the payslip. (It's based on Salary – Settings)
3. Before processing salary, use 'Salary Card' to update the value of Salary Head for the 'best employee of the month'.
4. You will also have to clear the increment for the 'best employee of the previous month'.
Hope this helps.