1. The execution of registered sale deed without receiving full sale consideration cannot be done instead you can enter into a registered sale agreement by giving the time requested by him to pay the balance of sale consideration amount.
Until then he can remain as a tenant in your property on a monthly rental agreement.
2. Don't take any such risk which may not be enforceable in law.
3. The best option is that you enter into a registered sale agreement and allow him the requested time for completion of execution of sale deed at a later date by paying balance of sale consideration amount.
The execution of registered sale deed without receiving the full sale consideration amount would be a risk and he may even legally refuse to pay the balance sale consideration amount telling that he has already paid the entire amount for which you to run behind court for recovery.
Hence better avoid risk.