it's a different way to manage prepaid credit.
A common way to manage it in many european countries, respecting VAT laws, is the following:
- customer buys 100 of credit; I invoice 100 + 20% VAT, customers pays 120, and has 100 of available prepaid credit
- customer buys service A , priced 10
I deduct 10 from his credit, process the order, and (if required by local laws) I issue this invoice:
service A 10
prepaid credit: -10
subtotal: 0
VAT 20%: 0
Total: 0
(if local laws do not require a "zero value invoice" to be issued, I can send to the user just a receipt/note/mail message/whatsoever, just to keep note of how his credit was spent)
- customer buys service B, priced 110
I deduct all his remaining credit (90), and issue following invoice:
service B 110
prepaid credit: -90
subtotal: 20
VAT 20%: 4
Total: 24
So user pays 22, and I go on releasing his service...
Cross checking:
- the user did two payments: 120 and 24 (total 144)
- I issued two invoices: 120 and 24 (total 144)
- I released two services, valued 110 and 10: total 120 (and 120 + VAT = 144)