What is the ruling on the father continuing his partnership with the maternal uncle in the truck, on the condition that the father takes 200 dinars monthly instead of sharing the revenue and expenses, and what is your advice to the father?

The initial agreement between your father and your maternal uncle to purchase the truck and benefit from its proceeds is valid, with no ambiguity or harm. As for the conversion of the agreement into a fixed sum after it was a defined, undivided share, this can be interpreted as a lease contract between the two partners for one's undivided share in the partnership. This is permissible, according to the preponderant opinion.