1. He has offered possession after 6 years. It is fit for possession so is it reasonable delay or extraordinary delay.
You hav to go through the sale agreement where it would be mentioned about the period of handing over possession after completion of construction.
If the delay is extra ordinary as per the agreement, then you may claim penal interest for the period of delay.
2. Is he justified in asking for delayed payment and that too in cash.
You can deny the payment and also ask him to give in writing so that you can give a reply in writing about the circumstances that led to the delay in payment.
3. I didnt understand point no 1 where you say i am liable for payment even if not mentioned in the sale agreement
You draft a reply through your advocate denying the allegations and the payment demanded, let him take legal action, which can be challenged.