We are working on it in the next release pipeline. Stay tuned. Temporarily, you have to count it manually and add a deduction payroll item "Salary Adjustment" to adjust the employee salary.

For example, the employee joins in the mid of the month. His basic salary is RM4500. After calculating, in the current month he is only entitled to get the amount of RM2200. 

Hence, you have to add a deduction item "Salary Adjustment" in the amount of RM2300.