@Libbpatt 

Am I understanding correctly that these cards are prepaid and the balance is reduced as they use the machinery?  Does the patient pay to load the card?

If that's the case, when the card is loaded, treat that amount as a liability because you have the patient's money but you owe them the service. Then, when the card is used, you will debit (reduce) the liability and credit (increase) income/sales.