Hiring Cost Calculation

Dear Power BI Experts,

When i am using the following function the actual salary does not picked up since the appointment date is in the month of any date therefore the acutal salary of the generated payroll calcuated accordingly the number of days paid in the month. I want the actual Salary on which the employee hired

First Gross Salary  = Calculate ( Sum(Payroll Sheet [Gross Salary]), Firstdate( Payroll[month])

Please suggest a dax formula to create a column so that i am able to find out the actual salary on which he/she has hired at..

Regards,

Saad Hanif