Who said anything about giving the manager 100% control over everything? Maybe you misunderstand what I said, so let me explain.
The ideal system is when a club defines the vision. Then hires managers and players who suit that vision, so that even with a churn, there always is consistency and good transfers are rarely wasted. In that system, the manager's influence in the transfer market can be limited. Based on last 5 years evidence, Inter HAS NO VISION. If you disagree, please do show me the vision that explains our hiring of our last 7 managers. Its fucking as random as it can be.
The best case scenario we can assume, is that Mancini was hired because the club either believed (and adopted) in HIS vision for Inter, or sat down together and agreed on a vision. If that is the case, then Mancini's vision and the club's vision are the same now. If part of this vision is for Inter to play with possession and width, then we must give the manager the wide players he needs. I am not saying that if Mancini wants De Bruyne, we must give him De Bruyne - the club is still responsible to manage the finances - but if he asks for a winger with certain characteristics, we must give hm a winger with similar characteristics (say a Perisic).
We also must define and adopt this vision for Inter for the longer term, whether Mancini stays or not. Which means that the next manager we hire should be an expert in playing with possession and width. So that all the investments Inter have made on players who suit this style are not wasted, and the next manager can start where Mancini left off and improve on the squad. That is how we can grow.
But the biggest danger is if we do not give our new manager the kind of players he wants. If Mancini wants a winger, and we tell him to use Hernanes or Kovacic there instead, that is disastrous. It brings into question why we hired Mancini in the first place for so much money. Because clearly we must have discussed and agreed all these things when we hired him... Inter must support their manager as long as he is working within the defined vision and within our financial plan.