Exactly, you just said my mind. There is nothing wrong for Liverpool to sell Quansah to Leverkusen who sold Wirtz to them. It's a matter of understanding and what both clubs feel is better for the improvement of their squad. Flick is a good coach, and I believe that he doesn't need Quansah that much and that's why he has to sell him.
Of course there is absolutely nothing wrong about that because the player they bought came from the same place so it would look like an exchange between two players from different teams to switch themselves, although the both players doesn't have any positions in common because for Liverpool Quansah play as a defender while Wirtz is a midfielder. However right now Bayern Leverkusen is in need of a player who does his best in defending because that was there affected areas last season, so Quansah can close the vacuum Bayern Leverkusen has been having.