Builder charging forced interest after disbursement and not ready give GST benefit

I was already in a 5 years (Jan 16 to Dec 20) Builders scheme with 0% interest EMI for which I had given 60 PDC cheques and payments were timely till Jan 2020. I also have builders scheme documented in email by their executive. But still I was getting demand letters with interest charges, which was deviating the agreed scheme and was facing mental stress from time to time. In Feb 2020 I have decided to go for a bank loan without availing scheme of 5 years with 0% interest. Provided that interest gets waived off before bank loan as proposed by builders’ staff verbally. I had demand letter (without interest) which I had requested by waiving up the interest and also have communication emails from builder's staff regarding waiving off the interest in demand letters. On the basis on those demand letters disbursement happened in Sep 2020. Since Feb 2020 the disbursement has been delayed due to Covid 19 pandemic situation and lack of cooperating builders’ staff for submitting the documents which is also documents in my emails Now builder has provided final demand where there is an attempt to recover the forced interest of Rs.53k out of which 40k adjusted against GST benefit amount as a waiver and in summary page it shows only direct part of interest Rs.13k which is misguiding the presentation of interest charged. I asked them to reverse both direct and indirect interest charges to agree and match with their provided final demand letters at the time of final disbursement. But they are not responding to the emails and verbally telling that not possible. Now after 97% bank disbursement builders’ staff is telling those demand letters were for bank loan and not confirmation towards the waiving of interest. This thing they are not ready to give in email. Sometimes they are telling verbally that I don't have GST benefit but not ready to give in writing. Now in builder's recent demand letters GST benefit offsetting transactions (Interest Charged vs GST Benefit) for Rs.40K dt. 31st March 2020 have been included. Those were missing in builders final demand letter at the time of disbursement in Sep 2020. I asked them why were those missing in their demand letters till the time of disbursement? Also asked them any reason for backdated adjustments i.e.31st March 2020. I have sent numerous emails, but they are not answering the emails. In short, As per the final demand they said 100% waiver but now they are adjusting against my GST benefit by showing backdated transactions. Also they are threatening verbally that they can charge interest till 400K if I force them to reply on the emails. I have everything documented in the email. Can anyone please advise on this?