You might want to introduced prepaid card to your customers along with post-paid card.
The post-paid card should carry a predefined credit limit that need to be maintained by the customers by paying for the outstanding dues on regular basis.
At the touch pos, the customer card needs to be validated and only then the operator is allowed to punch order based on available credit limit or prepaid balance.
Ideally a customer will either have credit limit or prepaid balance.
However if a customer is having both the prepaid balance and credit limit, the POS should consume the prepaid balance first and then credit limit.
Setup: In Touch POS Option set Activate Prepaid System = With Credit Limit.