Suppose someone has a $20,000 balance on a credit card or HELOC and settles for $10,000 - the IRS considers it cancellation of debt income and you are required to pay taxes on your $10,000 gain. Would there be a Chiyuv of Maaser on this gain?
I'd think it might be analogous to "performing kiddushin with a loan [that has already been given]" - i.e., where Reuven, who has previously lent money to Rochel, tells her that he'd like to use that money for kiddushin. The halachah is that this is not effective, because he hasn't given her anything now; the money is already considered to belong to her from the moment she borrowed it (Shulchan Aruch, Even Ha-Ezer 28:7, from Kiddushin 6b and Rashi there ד"ה אינה מקודשת). Since the borrower wasn't obligated to give maaser when he first took out the loan (as far as I know, money one borrows is not subject to maaser), then effectively there's no time when it would be considered income.
The Gemara there (and Shulchan Aruch, ibid. 28:10) does draw a distinction between the actual loan and the "benefit of having it cancelled"; in the latter case the kiddushin would indeed be effective (see Beis Shmuel 28:28, who says that everyone would agree to this). If I'm understanding Rashi (ד"ה לא צריכא) correctly, this "benefit" equals the amount that you'd have to pay someone to arrange the deferral or cancellation of the loan. So I guess you could argue that in the case you're asking about, if one made the arrangements himself and didn't have to pay a credit-repair firm to do so, then he might indeed be obligated in maaser for that amount - but not for the full sum that the creditor has written off.