Hello:
If you had a field on your form named Field1 then in the AfterUpdate event for that control you place the following code:
'
Private Sub Field1_AfterUpdate()
If Val(Field1) < 0 Then Field1 = 0
Assume your form has 3 fields Field1, Field2 and Field3. Field3 calculates the values of Field1 less Field2, enter the following formula in Field3
=IIF([Field1]-[Field2]<0,0,[Field1]-[Field2]).
The previous example using AfterUpdate does not work because the AfterUpdate event is not triggered when the field is a calculated field or updated by Visual Basic.