Booked a flat but the builder is asking to pay more

I have booked a flat last year December at that time rera for the tower didn't came so as for reference sales person showed us a sample area of another tower flat which was 1374 sqft 3BHK, let's assume the per sqft cost was 10000 now post rera it is come to our notice that actual rera of the flat is 1363sqft for 3BHK in this difference actual usable carpet is still the same wall thickness overall got reduced but due to which there is variation in cost we are constantly telling sales rep that we will be paying as per agreed to, but for 1363 that is 1363*10000 and not for 1374*10000 but they are asking us to pay the earlier ammount which increase prior agreed per sqft cost. I have already waited for almost 7-8 months now and I have given 10% of the amount and stamp duty. A Builder sales person saying there won't be any change in cost I am still adamant though to reduce it? But I am getting second thoughts what if they don't agree or cancel the booking? Is there any way I can get them to reduce this or has anyone faced this issue? This was our first home purchase and getting issues like this make me anxious.