He wouldn't cost 100m imo because 1) If Bayern want to sell him then cost will go down and 2) His injury history has to be taken into consideration.
Obviously I would take him back for if his price was in the 50-70m range. The only question is if the club can finance Griezmann, Lenglet, Arthur and Thiago in one window.
If we are to buy him, it's likely we would need to finance the operation through sales. But realistically how much are we going to accumulate for Paco, Gomes, Vidal? Maybe 50m if we are lucky?
Although rumors of the Jasper, Digne and Marlon sales would definitely make it plausible.