1) no payment should be made if flat owner is not willing to execute registered sale deed at time of making payment
2) if you make full payment and flat owner does not register sale deed in your favour you will have to file suit for specific performance to direct seller to execute rgd sale deed in your favour
3) ask builder to obtain no dues certificate from society that maintenance has been paid till date
4) letter from bank that full payment has been made of loan
5) original documents of the flat