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.
World’s longest fingernails finally cut after 66 years
The weight of his nails has left him permanently handicapped and in constant pain - he can not open his hand or move his fingers. A school yard incident prompted the challenge for Shridhar Chillal to grow his nails on his left hand when he was a teenager.
With V-for-Victory sign, rescued Thai boys celebrate freedom
Schnauer said he played "a small role" in the rescue efforts but the success of the operation showed what teamwork can achieve. The team's morale was hit when a former Thai Navy SEAL, Samarn Kunan, 38, died while helping lay out oxygen tanks underground.