Resale of flat in Mumbai, doubts

Hi all, I purchased this flat in Mumbai from builder, sale agreement was done and registered too. I sold the flat to a buyer who is availing loan from public sector bank,we entered agreement for sale and buyer got it registered by paying stamp duty etc 1) as per agreement balance payment for which he is availing loan from bank of India, was to be paid within 90 days from 11july 2016,I.e 11 October 2016,however due to goof ups in draft agreement loan process is stuck since last 3 months, valuation has taken just 7 days back. Don't know when sanction letter will be issued. In agreement there is no clause what if buyer does not recieved payment within 90 days. On basis of non receipt of payment can I as sellerback out of agreement, if yes agreement gets cancelled automatically or I will have to get it cancelled. 2) suppose I ignore the 90 days time period and wait for payment, after I get disbursement cheque, how to issue possession letter to buyer, is it mandatory if yes on stamp paper? Because possession letter from builder which I got is on letterhead and not stamp paper Thanks in advance