Raj,
The idea suggested by Avalon should work ok, except that assuming your
Fee field is a currency or number data type, the Expression argument of
the SetValue action would be 100 not "$100". Also, I would use the
AfterUpdate event of DateCompleted rather than the Exit event. And you
would also need a Condition in the macro like this...
[DateCompleted] Is Not Null
You can also do this in a VBA procedure, similarly on the AfterUpdate
event of DateCompleted, which would look a bit like this...
If Not IsNull(Me.DateCompleted) Then
Me.Fee = 100
End If