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)