Reduce Regular Earnings for Company Maternity Pay

Summary:

We have a requirement to reduce the salary of an employee at a different rate when the employee returns from Maternity leave in the middle of the month e.g. Say if an emp's Maternity leave is between 01-Mar to 31-Mar, then reduce regular earnings must happen at a rate which is based on calendar days so that the employee receives only Company Mat Pay and no basic salary. Similarly , if the employee returns from Mat leave on 10-Mar, then the system must reduce the salary at a rate which is based on working days. How can this be made