Not strictly true mate
They haven't agreed a deal that will rise to 80m at all Dave, they have paid 35m Euros and it''s been suggested that another 45m Euros in theory can be earned by the selling club IF the player hits every single one of the performance targets in the transfer.
From the Bayern Chairman for example
"The transfer fee for the current international, who was voted the Shooting Star of Portuguese Football in his home country yesterday is €35million, plus possible bonus payments that could become due if for example the player is named in the World Team of the Year or FIFA World Player of the Year."
The Likelihood is that it'll have stuff such a Ballon Dor nomination bonus, Ballon Dor winner, payments for CL title wins, a ridiculous amount of other goal, appearance and trophy targets that the player will likely never come close to reaching the majority of
Same way that Martial massive money transfer to United didn't cost them anything of the sort which was being reported and almost certainly never will do either.
In summary mate, Bayern won't pay out 60m quid up front on a player, it is completely against there very established and successful modus operandi