Need to recover 5200 what to do

I am an owner of company providing job work. We had signed a contract with some of the persons by a legal stamp paper. We give work for Free and ask them if they cannot do the work or cannot Complete the work properly as per contract terms and condition we will charge 5200 as a utility charges from them. Which we signed a contract with them on a stamp paper by email. We also have a call recording for the same for the customer. Some of the customers dont send me the amount so we send them legal notice Via our company advocate But yet they donot pay the amount. Kindly tell me what to do there after..