URP demanding money back, can I deny GST component?

Dear Sir or Madam,

We are into AMC of a domestic appliance, an URP customer who signed an AMC with us in Dec 2022 wants to cancel the contract & demanding refund for 9 months.

We are okay with it & have agreed to refund on pro-rata basis.

However, I have said to the customer that GST won't be refunded as we have already deposited the same, but the customer is demanding refund including GST.

What should I do?