Since there is an existing sale agreement, without cancelling the same, the seller cannot venture into selling the same property to a new buyer.
Whether the previous buyer is a gent or buyer, the question is that he has entered into a sale agreement with the vendor, hence if the proposed sale with you goes ahead, the previous buyer will file a suit against the vendor and bring this property into dispute and he may even implead you as a necessary party to the suit.
Thereby you may lose your peace after buying the property and you will be frustrated and would be under stress till the legal dispute is solved or disposed by court of law.
Before that you would have spent a lot of money, time and energy towards this litigation/court case.
Do you want to undergo this stress and hardships by desiring to buy this property which will definitely put you in trouble immediately after you buy it.
You may obtain a legal opinion from a local lawyer before venturing into investing into this asset.