1) you should register your sale agreement with builder . Pay stamp duty and registration charges
2)then sell your flat to purchaser
3) in the alternative cancel your agreement with builder . Take refund of money paid by you
4) then builder would sell flat to the purchaser
5) in case of tripartite agreement banks are reluctant to sanction loans as it is regarded as an exercise to save on stamp duty and registration charges
6) in case of tripartite agreement consideration to be paid to you can be mentioned