I'm relying here on what I consider a common-sense tautology: A rule that says that your traits may not be higher than X prevents your character from having traits higher than X. Extending the premise far enough to suppose a Storyteller would let you buy a trait higher than X under the presumption that temporarily raised Trait Maxima would permit it is, I think, generous enough.
As you posit it:
- A vampire of the Eighth Generation uses Blood of Potency, garnering enough successes to lower his Generation to Sixth.
- Having raised his Trait Maxima to seven dots, that vampire spends (as an example) 30XP [or 36, depending how you reckon "current"] to raise his Auspex to 6.
Now, eventually, the effects of Blood of Potency wear off, lowering the vampire's trait maxima back down to five. It is self-evident that you cannot use dots that you cannot possess, and there is no rule allowing for those dots to somehow go "dormant" until you raise your attribute maxima to a point where you could make them legal again. Therefore, the result would seem to be that your traits are reduced to beneath the new maxima, and the experience points to purchase them lost. ("Sanctity of Merits" doesn't seem to apply here, as Disciplines aren't Merits.)
Alternately, a Storyteller may decide that such dots go dormant, like those of a ghoul who has not been fed with blood for some time (no longer than half a year, then they start to decay). However, given the ease with which Blood of Potency can be invoked, this is an unlikely and probably ineffective workaround.