How to close the unfinished sale agreement

I signed an agreement to sell my 3BHK apartment (in Faridabad, Nehar par area). This apartment doesn't have any outstanding loan. I purchased it directly from the builder. The flat is still under construction and we have not yet got the possession from builder. As per the 'Agreement of Sale', Purchaser and I agreed to pay me the agreed selling price in 3/4 installments - across total period of 3 months, last of which I should have received by Nov 2021. The total deal was 28Lacs and the purchaser paid 6 Lakhs as advance in 2 installments but failed to make the complete and final payment, though. Its been more than 8 months since the deal's last date but the purchaser is not cancelling the deal and not completing it too. They first made excuses and requested more time. Now we have been following up with them to close the cancellation at least. We wanted to cut some amount of token and return back the rest but they want complete amount back. We wanted to meet and negotiate but they are not responding properly. They look to be property dealers, and we are a little skeptical of their intention. Questions: (1) We want to understand the scope of legal issues in case they don't cooperate and in future we try to make a sell deal with another party, or at the time of offer of possession from builder to us - would they have any right to claim on property? (2) Do I need to return the complete advance amount now when the full payment wasn't done in the due timeline by the purchaser? (3) Do I need to send a legal notice to this purchaser that he won't have any stake in sale of this apartment now? (4) If such a notice is to be sent, will it need signature of the purchaser as well for acknowledgement, or just sending a notice is enough? 5) What will be the cost of such sending such notice? I'd like to have a discussion on this over phone (I am in Bangalore right now).