There are several layers in getting anything register
1 execution
2. Presentation
3. Registration
Since 1st step of yours is not complete you donot have any right to get the instrument registered
However if your father have registered an agreement of that period things will become lighter.
I would suggest get the valuation of the property done of the same.
If it is more than 30 lakhs then it would be good that you pay the rest amount only through a post dated cheque or DEmand Draft , and ask him to get the property registered
Bcoz if the valuation is more than 30 lakhs u directly have to pay Capital Gain tax of 30% on the same to the incometax dept.
And sell off the same on a higher price to the third person