Should I sell or not and how to maximize profits?

This might sound crazy but I want to sell my top striker who just had the season of his life. Namely Dzeko. He is 31 now and just made an amazing season winning the Golden Boot and a few other prizes. He loves the club, doesn't want to leave and all but I just need that his time is up and now is the time to profit from him.

He is valued at 19mil. but by offering him to clubs for 40 mil (euro), I get offers at 30. Is there any way to increase this? Any trick to make them pay more? I tried offering first for 90 then went all the way down to 40.

I should note that the transfer window does not open until a week or so (it's the end of the season).

So in short - is it a good move to sell him now? or should I keep him. (Just to note I have a 24 old amazing striker + 17 wonderkid who covers Dzeko's abilities pretty well, as well as now happy wayne rooney with crappy stats but at 31 and of course David Villa at 35 )

And how do I get most out of a possible transfer?

Thanks!

Cant you get money from the amout of goals he scores and games he plays aswell as the 30 million?