But if his agent is going in and asking Txiki for parity with KDB's contract, I'd be asking some very searching questions before I'd offer anything like that.
1)He doesn't have an agent.
2)He's not asked for parity with KDB
3)The club are the ones looking to tie him down.
If he did have an agent however I imagine he'd sit down and put something like this on the table -
Since Sterling signed his last contract -
Goals and Assists for Manchester City -
Sterling - 100
Jesus - 88
De Bruyne - 81
Mahrez - 79
Aguero - 75
Bernardo - 60
Gundogan - 46
And then something like this -
Games started for Manchester City
Sterling - 130
Walker - 126
Bernardo - 122
Gundogan - 111
Mahrez - 110
KDB - 99
Rodri - 94
Fernandinho - 91
Jesus - 89
Laporte - 88
Aguero - 79
Stones - 76
So once he established he's the most productive player at the club, and Pep's most played player. He'd probably then point out he's 3rd in line to the England captaincy and he's been elected by the players as one of the club captains, so his position at the club as one of the leadership group is incredibly clear.
Then he'd probably mention his big endorsement deals which bring in loads of money for the club who own his image rights, his massive public profile and awards.
Then they would all reminisce about the 5 trophies Sterling helped the side win since that last contract, talk about ones they hope to win in the futures and Soriano would hand over a 5 year offer for a decent bit higher than his current deal.
Becuase it's really that simple. No club ever doubts re-signing the player who contributes more goals and assists than anyone else in the side. If anything, Sterling's drop in form has come at a very handy time for Soriano, because if he'd scored another 30 goals this season then he really would be looking at KDB levels of money.