Auto-Enter calcularion for "charge 1" :
terms 1 = "A" ; 0.01 ;
terms 1 = "B" ; -0.01
Note that it is possible that I haven't understand correctly your question.
It would probably be better if you have your charges (with their terms) as a related lineitems table. What if you want to add an additonal charge? If you ever do, you will have to change many things if you keep down this path. Also, can charges be added on different dates? You cannot track that fact (as it stands) because you don't have a Date1 and Date2 field. Whenever you see this type of situation (particularly on something so ever-changing), it is best to use a related table. Design will be much simpler as it grows.
Anyway, I wonder if you are indicating that if Terms = "B", the charge should be negative (whatever the charges). But we would need business rule ... what if the charge was entered as a negative? Would you want B terms to REVERSE the charge whatever it is? Or, more probably, keep the charge negative if B? I will guess that, even if User properly entered a negative number, you would wish to keep that number negative (implying that Terms B means Credit).
Please clarify ...
The problem I see is if the User changes the terms. If you apply auto-enter to the charge field then you must uncheck 'replace existing value'. If you always want the charge entered as simple negative if Terms = B (assuming some Users might forget to include a minus) then try this auto-enter (replace) on the charges 1 field:
n = Abs ( Self ) ;
Case ( terms 1 = "B" ; n * - 1 ; n )