registration of sale deed

The seller has no right to re-sell the property already sold and registered to A. He cannot pass you the title or rights which is not vested in him, in future you will not only face legal cases but also loss of money, hence, it is not at all advisable to purchase such property.

Not a goos thing for sale

When there is already sale deed, the matter ends there. Even if mutation is done or not it is of no consequence. You will certainly face probelms if you buy. "Purchaser Beware" = "Caveate emptor"

I thins the querist is not realisitic but an imaginative for time pass. Once the title of plot is in favour of A and if thereis any lawful dspute between seller and A then also the pendancy of  the possession and mutation entry has no relevance once the Title of land is with  A by registered documents before 20 years which holds valid for the years to year unless some contrary is proved. If  to chose to make transactions  for such  property the buyer will  face court matter civil case as well as criminal case from the private complaint filed by  A under provisions of IPC.