There is a really simple solution - a player's agent should be paid by the player, out of the players money, on a %age basis. Or, there should be a standardised formula (most likely a standardised %age). Pretty much any other system creates a conflict of interest. It's almost unheard of in any other walk of life for the agent of the 'talent' to be paid for by the hirer of the talent, pretty much whatever figure the agent plucks out of the air
The agent in this instance can name almost any figure they want. Let's say £10M. If United say yes and we say no, the agent will as you say do everything they can to force a move to United. Is the player even made aware ? There is insufficient regulation to be sure. THe system is geared to the benefit of agents, to the detriment of all the other parties. The system is inherently wrong and broken but the agents seem to hold too much power for the clubs to do anything about it.
The way it should work, in an example like this where a low transfer fee & high wages are paid because there isn't much left on the contract, is that there is a large signing on fee and the agent takes a percentage of that. If that was the case, which it would be in any other industry, the agent would be working on behalf of his client (the player) to get him the best deal possible which in turn would him the most as agent.
It's not rocket science. But there are no real signs of any major shifts in stance looking likely
What you would like to see, if this is one of the main stumbling blocks with Sanchez, is he turns round to his agent and say you will get whatever you get from the club I decide I'm going to. But does he even legally, have that power ? Or can the agent stand there and say if city dont pay me, the agent, what United will, I won't sanction the move ?