Dear Dharam,
In my career as a State Government Official for about 33 years, I worked as a Deputy Controller of Legal Metrology for about 8 years. Though it is long back falling in the early 1990s , still I do remember how the retail outlets of Petroleum Products have been run with the employees and particularly in big cities where the sales are very high during peak hours, the conduct of sales and service in an orderly fashion would be very difficult for employees as well as supervisors/managers who supervise them. Therefore, in most of the outlets, pump operators/salesmen are permitted to collect the cash from the customers directly and finish the transactions then and there during their shifts and at the beginning and end of the shift they calculate the total sales during their shift from opening and closing meter readings of their respective dispensing units and hand over the total cash collected to the cashier. When sales are effected through card swiping, the retailers' copies are kept by them and submitted along with the cash at the end. If the card was swiped for a higher amount, the money would have come to the establishment only. Therefore, the only chance of fraudulent practice could be selling the proportionate quantity of the product to the value of the excess amount collected to some other customer without bill. Therefore apart from the CCTV footage, you ought to have verified the total quantity sold by him during the shift and the total cash remitted by him adding up the card sales effected by him on that day. Then only, you can come to the conclusion of misappropriation or theft.
This apart, from your further post what one can easily infer is that there is no effective control over the employees in your establishment. Union of employees can not have a say in the matter of maintenance of organizational discipline. Unless and otherwise there are some loose ends in the method of sales and service, payments to employees and the manner of over all work discipline as well, no possibility of tolerating such misdemeanors and unbecoming conduct of an individual employee.
Be firm in the matter of maintaining discipline.