That holding company, Exor, owns 64 per cent of Juventus, and 30 per cent of FiatChrysler Automobiles (FCA).
Juventus' supporters show Cristiano Ronaldo's official Juventus jerseys.
That's money that could have been used for job-boosting investments such as making better cars, the union said.
"It is unacceptable that while management continues to demand huge sacrifices from (Fiat Chrysler) workers over years, the same management decides to spend hundreds of millions of euros to buy a football player", the union said.
Ronaldo and Messi have battled to be deemed the best player in the world for the past 10 years, winning five Ballons d'Or apiece as Real Madrid and Barcelona fought for supremacy in LaLiga.
The powerful Agnelli family including Juventus' president Andrea Agnelli co-own the Turin-based club and Fiat.
The USB union has called for a strike at the Melfi plant in southern Italy, which makes cars including the Fiat Punto and the 500X.
Liverpool FC finished in fourth place in the Premier League table last season as they secured Champions League qualification for next season.
As captain of the Portuguese team in the 2018 World Cup, he scored four goals, including a hat-trick against Spain. Because Juventus and Fiat Chrysler are partly controlled by the same owners, the announcement has triggered a union strike at Fiat Chrysler's Melfi plant in Italy.
Cancelo has joined the Bianconeri in a €40m deal from Valencia - and was soon followed by former Real Madrid star Cristiano Ronaldo.
Canada 'caught in the crossfire' of US-China trade dispute
The first round of tariffs covered Chinese products ranging from farming plows to machine tools and communications satellites. China said it is "shocked" after the USA announced plans for fresh tariffs, escalating a trade war between the two countries.
Donald Trump Publishes Letter from North Korea’s Kim Jong
Pompeo denied that the talks went poorly, telling pool reporters that progress was made "in every element of our discussions". Although the letter was written less than a week ago, it's not clear where US-North Korean talks stand now.
This Is the Most Tender Photo of the World Cup So Far
Croatia will meet France in Moscow on Sunday after they came from behind to beat England 2-1 in the semi-finals on Wednesday. And his prediction for Sunday? Crowned with the words "Sir Harry", it is cradled in the sentence: "World Cup Winners 2018".