Runwal Group — Demand for additional amount

I have tried to contact the right representatives from Runwal but to no avail.

Summary:
I have bought a 2bhk flat in Runwal forest - T4, Oak building, 3401 in the year 2019 March. The agreed price which the sales team committed, signed and shared was 1.69Cr all Inclusive. The agreement value is 1.5Cr. In the cost sheet 5% GST was billed to me and the deal was signed off.
I have done the agreement basis this price, taken loan basis this price and have been paying to Runwal as per the completion schedule. Never ever defaulted in my payments and have been paying in time as required.

Now at the time of possession when I was about to pay the remaining 10% of the balance payment, Runwal finance team came up with an ask of additional 12lacs over and above the agreed value. So now the total flat price comes to 1.84Cr, instead of agreed and signed value of 1.69Cr.
The Runwal finance team is saying there was a mistake from Runwal side and I need to be paying additional 12 lacs. Inspite of having the signed cost sheet and the documents they are asking for additional amount. This is very unfair and is an huge amount. Moreover, none from the finance team is discussing anything with me to close this issue.

The sales team has washed their hands off this issue and has left me in the lurch. Not even a single person from Runwal is forthcoming and explaining us the rationale for this. They are giving us an ultimatum that we do not have a choice but to pay this amount.

Inspite of having all the agreed and signed documents they are threatening to pay additional 12lacs.