It's practically impossible. He just transferred to Galatasaray, and we know that such a thing wouldn't be possible after signing a contract. Sane is a quality player, and if this injury had happened beforehand, Bayern Munich might not have wanted to sell him. I'm sure they would have agreed to a contract extension with him. However, Bayern Munich's injury after signing with Galatasaray makes me think they regret not signing Sane. I'm sure Kompany regrets seeing a player who knows Bayern Munich and Germany intimately leave the team. Now they'll need to sign a new player, and to do so, they'll need to scout a few players.

What you said is true, because every player has a contract with the team that signed them, so no player can just leave to any team they like if they're still under contract with their old team. However, being loaned to another team is different, as that team may already have more players for a particular position, so it wouldn't affect the loan move. I've also seen no news about Leroy Sane returning to Bayern Munich after being signed by Galatasaray last month.