It appears that the smart fellow understood the flaw in your process. After every credit or cash collection, there should have been reconciliation by the back-end office. This was not done. Secondly, in this age of online banking, what was the need to authorize the field staff to collect the cash?
When the field staff took some company property outside the company premises, how many pieces did he take, and how many did he return to the back office? Why was daily accounting not done? When courier companies hand over the documents to the courier for delivery, every evening the supervisors check how many documents he had taken and how many undelivered documents he has brought back. The supervisor checks the Delivery Sheet of each courier. Did you have a similar arrangement?
You can lodge a police complaint. However, you need to have evidence of misappropriation of the funds. When the retail shop owners (retailers) gave some money to the field staff, did they take any receipt from him? If yes, then it can become evidence.
Above all, have you taken any agreement from the field staff about cash transactions? Couriers or delivery staff of e-commerce companies also collect cash from their customers. However, they are authorized to do so, and an undertaking is obtained from them on when the cash should be deposited, with whom it should be deposited, and in return, what receipt they will get.
Anyway, lessons are always learned the hard way. Rs 15,000/- is not that big an amount. I hope you improve the internal processes hereafter.
Thanks,
Dinesh Divekar