Charged in GBP instead of local currency

Hello, I used Monzo in India on 5th Feb 2020 for a very large transaction. I repeatedly asked the merchant to charge in local currency which is INR. However, it looks like something went wrong and the bill says that it got charged in GBP from their end (which they are refusing now over phone, they are saying it was in INR only). When I put my pin it was showing INR on the screen. After the transaction, Monzo showed one amount on their app, however after presentment and settlement, it’s showing £110 more after 1 day. In this process, I have now lost approximately £110 because of this. The retailer bluntly said they can’t do anything about it. I have raised it with Monzo, not sure what to do now. Please let me know whether anyone faced this issue. I feel extremely distressed and cheated.