My sign was taken on few blank delivery challans and invoices by a?
Delivery challan would be duly filled with the details of the product supplied to you. Am sure that you mustn't have acknowledged delivery of NOTHING! As you said that product supplied to you was faulty, means that you acknowledged the delivery of that product, may be without inspecting the same. in case that vendor waves those delivery challan in the court, you can safely take the defense that it only proves that the product in question was delivered to you and no more. You were not given time to check the product and hence you are in court!