Firstly, I do not profess to be an expert. Ideally your first port of call is your bank. Try to proceed in the transaction as follows:
Firstly, you should obtain a letter from your bank stating the total loan that is outstanding in your name for the said flat. This will tell the buyer what are your dues (as well as the interest rate on your loan so that the buyer can then estimate your total dues as on a particular date in future). Your bank should also state in the letter that they note that you are selling the flat and that the buyer is going to settle all outstanding dues from you. Your bank should further state that they will not permit any refinancing/further advances against the said property from the date of its letter. This will give the buyer/buyer's lender assurance of your liabilities secured by your property. Take three original copies. Give one to the buyer, one to the Society and one for your own records.
Now request the Society to issue a letter (not "NOC" - how can Society say it has no objection?) saying what are your dues to the Society (if any) are and also the name(s) of the lender(s) who have an interest (i.e. charge) over your property as per their records.
So now the buyer has to two letters which he/she can then show their lender. This is all the buyer needs at this stage.