my case is

if one of our employee get incremental with-in month for-example 

Employee Salary is 500$

he get increment 100$ during the month
Salary Jun1 - Jun 15 = 500$
Salary after Increment = 500$ + 100$ = 600$
Salary after Jun 15 to onward is 600$
so Salary end the month of Jun should be = 550$

and onword is 600$

how i can handle this

can i create new Contract for Employee ? with new salary rule ?