The property was transferred to your father as per Income Tax Act when he paid full consideration and the PoA was given in his favour. The seller ought to have offered the profit on sale of such property at that time itself.
Now your father can get the property registered in his name as per the Agreement to Sell.
The seller has no additional tax liability on registration now. As he ought to have offered the capital gains in 1987 itself, after receiving the full consideration. As he did not do so, it is income escaping assessment and hence assessment may be re-opened u/s 147/148. However, the proceedings u/s 147/148 are now time barred.
If the PoA is irrevocable and all the rights were passed on to your father, then the transfer would have been complete in 1987 itself. Otherwise, there will be capital gains now on registration. So the PoA is an important document that determines the tax liability now or then.
If the sale is now considered to have take place, the stamp duty value will be considered, if the consideration is less than the stamp duty value.