Part of his job is to advise. He may mention state tax rates, advertisement opportunities, quality of life and other factors that go into choosing where to play. Now, if he's pushing a player towards a team just for his own personal gain, such as kick backs, owed "favors" or the like, then yes, he is not looking out for the best interests of his clients, but much of this is gray area. He may think money is the "best interest", and if he's pushing his client to take a $210M deal over a $200M deal just so he makes more commissions, but fails to tell his client he'll be paying $20M more in taxes, then he's doing wrong.
I'm not sure I ever heard of examples like that.