Sales Commission Rate Problem

I have a master form named TimeCards, and on it are many fields.
One is a combo that looks up the Employee and his commission rate
It is named SalesEmpID and gets its info from a table/query with 2 tables in it.
Employees and TempOrCon

I also have a bound control on the form which gets its data from the Time table and it is named SalesCom, it has no set default other than 0.
SalesCom is used in the payroll function of the database to calculate the employees commission for that particular payroll time period.

I am confused as to what to do here. I cant really change the control source of SalesCom since it will throw off my payroll.
I tried using the default for it with =SalesEmpID.Column(2) which comes from the combo lookup named SalesEmpID, I have another unbound textbox named
TextBox500 which stores the value of the combo sales commission rate
=SalesEmpID.Column(2)

Any suggestions as what to do so that SalesCom will use the value that SalesEmpID looks up and not change its control source?

Attached Images

TimeCard.png (209.1 KB)