RegularUser BO. This shows the fields and sample data:
Name Jaymer
Initials Jay
DisplayCalc Jaymer (Jay)
The Calculated field is from a Rule Formula.
On a Job screen, the user picks the Salesman using the DisplayCalc field, that displays CHOICES from the user table.

Then, this rule fires, to lookup the Record and store the Initials (or ID# in your case).
If Job.SalesmanDisplay WAS CHANGED Then
FIND RegularUser WHERE RegularUser.DisplayCalc = Job.SalesmanDisplay
Job.SalesmanInit = RegularUser.Initials