Police called to the scene in Phrae province found Prayoon Bamrungjit, 62, dead in a shack with a severe wound to his head.
Theerawut Chanmuen, 39, was also at the scene, waiting to confess his crime to police.
He told them his father-in-law was an addict who was desperate for money to buy drugs. He had tried and failed to sell a family plot of land and was attempting to sell a cow instead, Theerawut told police.
The pair started arguing over the matter and Prayoon became enraged with his son-in-law for disagreeing with him. His father-in-law grabbed a knife and charged at him, said Theerawut, who confessed to then hitting Prayoon five times with a shovel, killing him.
He was initially charged with intentional murder, though his family say he might act in self-defence.
Theerawut’s wife said she did not want to speak about her father, but added that her husband was not a hot-headed man.