See, in what is now the United State of America, we had over a century tradition of slavery, where humans owned other humans and the American sports landscape is still quite sensitive about that in this day and age. especially because the two biggest sports, NFL & NBA, are comprised of predominantly athletes descended from slaves.
“Owning” players and selling players is a third rail.
So instead of saying “We are selling this player.”, American sports act like everyone is in on it and you are getting players from different teams to agree to do a work exchange, kind of like Foreign Exchange Students, where one team welcomes a guy into their home in exchange for another person getting to go experience someone else’s home. So contracts may or may not be renegotiated because it is all just a glamorous work exchange among millionaires.
Early in the 20th Century, post Reconstruction when the Klan was rising to power, lynchings were common, and no one gave a shit about the feelings of others, player sales did occur in American sports. And the ne’er-do-well son of impoverished working class German-Americans nicknamed “Babe” Ruth was sold by the Boston Red Sox to the New York Yankees for $125,000, so it did happen in the USA sports landscape at one time. And when Ruth made it to NY, he signed a new contract for a significant pay increase… like what happens today in Europe.