The main question is, does the club afford to pay that much to him? If yes, then I don't see a problem here. As long as they are getting one of the top players in the world back into the club, they are okay with the price they have to pay because they know it's worth it. They have seen that the club hasn't been the same since he left, so they had to get him back somehow.
We don't know what happens on the inside, but a player of his caliber surely deserves an amount like that. These clubs earn a lot of money from multiple sources, and they wouldn't make a decision like this if they felt it was too excessive and can cause them financial problems.