Around 30% of Fiat is owned by the Agnelli family, which also owns Juventus.
The union also mentions "huge economic sacrifices" being made by workers at the same time €112m is being spent on Ronaldo.
The reigning Serie A Champions, Fiat Chrysler Automobiles (FCA) and CNH Industrial (CNHI) are all owned by a holding company Exor and a trade union prompted Fiat workers to down their tools.
The powerful statement further explained how the company ought to invest in vehicle models that "guarantee the futures of thousands of people" instead of spending exorbitant amounts on a single football player.
The strike will take place at the Melfi plant, a city around 100 miles from Naples.
According to analysts, Bloomberg reported, Ronaldo could give the auto manufactures an advertising boost: the striped Juventus shirt has Fiat Chrysler's Jeep logo on the back.
The strike will begin at 10 p.m. on Sunday and continue until 6 p.m. the following Tuesday.
Spanish soccer giant Real Madrid Football Club agreed on July 10 to transfer five-time Ballon d'Or victor Ronaldo to Juventus after the player asked to leave. The deal could be worth €300 million ($349 million) when wages are included.
Born on the Portuguese island of Madeira, where the airport is named after him, Ronaldo has had a successful career at Real Madrid, scoring 450 goals in 438 games for the club.
The likes of Michel Platini and Zinedine Zidane were great players when they first arrived in Turin but had developed at Juve before becoming arguably the best in the world.
His move caused Juventus' stock price to rise by nearly 40% when the news broke.