I’m not an expert on contract law, and I have no inside knowledge of this deal, but logic tells us a few things.
1. Rangers wanted to sign the lad, and GVB told him he wanted him for the first team. So why a loan deal? Surely not a “try before you buy” in these circumstances, and given what we know already about how he can play? Logic suggests Rangers couldn’t afford a permanent purchase of Tillman in the current window, and not because of his salary, which Rangers must be paying anyway, but because of the agreed fee.
2. For an “exclusive option to buy”, a fee must be agreed.
3. Therefore, the fee must be at least £4m, perhaps significantly more. More than Rangers could afford this window, but not more than Rangers think they can afford next summer.
4. If a contract has been agreed between the clubs, no other club can come in with a big offer - Rangers have the rights to first refusal, at the end of this season. Ergo, Tillman is guaranteed at Rangers for this season at least. But the lad’s head could be turned by a big money offer from a big club, so he could still choose to go elsewhere at the end of the loan.
5. Nobody knows the buy-back amount, but it must be to prevent Rangers buying Tillman for, say £5m this season, then selling him a year later for £25m. Bayern are no mugs.