How does one pay off flaws?

Question

The description of the Sterile merit in M20 Book Of Secrets reads as such:

For some physical reason, you’re unable to sire or conceive children. Whether this is a Merit or a Flaw depends upon whether or not you want to sire or conceive children. Generally, this sort of condition can be easily cured with a little Life-Sphere magick; in your case, though, it can’t be rectified until and unless you, the player, discard the Merit or pay off this Flaw.

But there are no mechanics provided for paying off flaws. Are there any such mechanics in another book and if so what are they?

Answer 1

Storyteller fiat and experience point expenditure.

On page 642 of the 20th Anniversary Edition Core rules for Mage: The Ascension, it reads:

Generally, Merits and Flaws cannot be purchased after character creation, although the Storyteller may decide to give them to characters in place of experience points if those Traits seem appropriate to the events within the chronicle. Depending upon those events, a player might be able to use experience points to buy off a troublesome Flaw. In all cases, Merits and Flaws are optional Traits. The Storyteller may disallow some, all, or none of them – it’s her choice.

And then, on page 645, we find:

Flaws offer extra freebie points during character creation. Even so, they’re not only point-magnets; each Flaw presents a story-based challenge that must be faced and overcome throughout a character’s life. Until and unless that Flaw gets bought off with experience points and some significant events within the chronicle, a Flaw remains a monumental pain in the ass for a mage, his companions, and the player who decides to take it.

So, you need the ST's permission, an in-fiction trigger, and a spend of XP. How many XP? That question isn't answered in the experience point tables in M20; check with your storyteller.