Dodgy ground comparing footballers with slaves, mate.
The difference is in my job I'm not an asset worth tens of millions of pounds to my employer (debatable to be fair).
If you owned a factory and I owned a factory, and I came and bought your best machine, and you didn't want to sell, you'd feel pretty aggrieved if I just bought it anyway.
You'd be even more aggrieved if I somehow snook in to your factory at night and stole the keys so it wouldn't work again until I'd bought it off you.
I don't think footballers are just enployees, they're multi million pound assets as well. In US sport, players under contract are traded to teams and cities they have absolutely no say in. They just turn up to work in LA one day and get told "you've been traded to Denver" they pack their bags and off they go, no say in the matter.
The concept in the US is that the clubs own the economic rights to the players and they can do whatever they want with them. Trade them like Panini stickers.
In Europe it seems too far the other way - players hold all the power. If they want to leave, they essentially go on strike. I believe in the right to strike for fair working conditions, but Sanchez is on £150k a week to play football, his working conditions are pretty favourable.