There is no way Bayern can let them win, that's obviously not allowed in football, the only thing you can say is that, Bayern will prolly field a weaker team to give other fringe players some game time as they have already qualified and topped the group as well, thus if that is the case, Barcelona then automatically have the advantage, but we'd have to see the team list for Bayern first.
Having said that, all the pressure will be on Barcelona, they can't afford to drop to the Europa league, it'll affect their club reputation, they'll be hoping Dynamo Kyiv puts up a fight against Benfica and they themselves get the result against Bayern, it's really going to be an interesting final matchday in that group, and the final outcome could have real consequences.
The best case scenario for Barcelona would be that Bayern Munich would rotate their squad in this match. Because they are already very comfortable and they are going to play a Bundesliga match by only a two-day rest. This seems to be a quite probable scenario in my opinion. If this happens, Barcelona have to make use of this last chance in the right way. Otherwise they will have to say goodbye to the CL.