The way he played this season.. his effort and focus was pathetic, so I'd all for selling him for 45M +.
But there are not many options out there, and I think it's understated how vital are quick and offensive fullbacks for elite teams. Overlapping = spacing, which is the most important aspect of today's game.
So I'd rather keep him unless there is a real will to invest on someone good for the positon. Bernat looks like the perfect fit (haven't see him play in 2 yrs though): contract running out, excellent relations with Bayern, player not demanded on the market right now.
So if it was Sandro 45M+ for Bernat 15/20M I'd be good with it.
But for god's sake, DON'T BUY DARMIAN to be a starter.
He actually could be useful as a backup though, with Spinazzolla as the fringe 5th player of the position (LB: Bernat, Darmian; RB: Cuadrado, DS)