Hospital fails to give cash receipt for the claim? What to do next?

A famous hospital approved by the Govt. of Tamilnadu in Kumbakonam failed to issue the cash receipt for the amount paid for the treatment. They said the claims and reimbursement is not entertained. Instead they gave the details and the amount paid in their letter pad with revenue stamp sticked and sealed and it is signed by the Doctor and doctor’s seal. The insurance company is saying that they need the cash receipt. Kindly help me what to do next.