TBH, pay the 45 mil. We're in no position to negotiate for world class (or young almost world class) players, we should feel lucky they still want to come here.
Even if he fails I'm sure we'll be able to sell him for 25-30 mil €, and if he doesnt and does well we'll probably sell him to Real or another club in a few years for the fee we paid for him.
In either case, we need great players to get back to the CL and attract other great players, we're not in a position to negotiate about 5 mil +/-