If it doesn't work out we will have to get Werner. Lewa, Werner, Coman, Gnabry is OK. We won't realistically be challenging for the CL for a few years either way.
Neither of us know how damaged the relationship between City and Sane is. If it's just a case of having his head turned by the idea of being a Bayern player he should get over it and sign a new deal. Then it's the perfect result for you.
If there is a more serious problem then surely it makes sense to take 80 million for an unhappy player which you can then spend on someone else without everyone screaming FFP at you.