Student or Learner
The context is a trading business setting where "A" paid to "B" more ($200) than the invoice total($120) for the previous purchase order, creating some balance($80). This time "A" gives another purchase order, invoice total of which comes to $100 this time. In this case, "A" may only have to pay $20.
In above context, as in the following sentence, the shotfall in the invoice total makes sense to mean $20?
- Payment will be made for the shortfall in the invoice total.
That's a very confusing way to say it.
After applying the credit from your prior invoice, the amount due is $X.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.