we buyed him 100% and then sold 50% to Sampdoria. which means he is our player but owned 50% by them
for example, Immobile was sold to Torino 100% and then we buyed back 50% so he was their player and they had the last word in his selling
They didn't have the last word in selling, or more correctly, we did as much as them. Both parties need to agree to a sale. With Immobile, Torino was just much more reluctant to sell him/valued him at a higher price, while we weren't really objecting to a sale.
What you guys are probably referring to are the playing rights, which basically determine which team can choose the team the player currently plays for.
E.g.: Us and Sassuolo both own half of Berardi -> Sassuolo has playing rights -> He plays for them.
Last year: Us and Genoa both owned half of Boakye last year -> We had playing rights -> We decided to loan him to Elche.
But playing rights don't mean that any party has more power over the player from a transfer perspective.