Query regards a property

Hello,

My grandfather purchased a land around 20years back. The deed or registration documents had a statement that the propert does not belong to wakf board. Now only recently(a week ago) the son of the property seller has come to us stating that the property belong to wakf board and that his father sold it without knowing about it.

So, we are now in a state of confusion. Can someone provide us the thought on how to find if the property does belong to wakf board or not?

Thanks.