In football/soccer, what's a mercenary?

A mercenary is someone who is in it for the money. It is usually used for soldiers. When the United States was breaking away from England, England paid a bunch of money to German soldiers to fight with them. They had no reason to be there if they were not paid.

This term then got moved to describe a football player. Fans want to think their players play for the history, the jersey, the tradition of the team. They really play for the money, so when a player leaves a team to another team (especially for a higher pay check) they are seen to be only playing for that new team for the money, just like mercenaries only fight for the money.