Is the son entitled to claim land that his father sold 18 years ago, on the pretext of an unfair price, and would I be sinning by fencing and registering the land, knowing that my father affirmed the validity of the sale?

The son's claim is not considered valid as long as the father has acknowledged the sale, even if the sale was for a low price or a gift. The land belongs to the buyer, and he has full discretion over it. It is recommended to reconcile with the paternal cousin to preserve affection and kinship ties.