I'm going to put my 2 nuyen in here. I am of the opinion the quality is very overpriced for the amount of cash the character receives. The idea of buying it off with karma seems a bit silly, as after paying off the total debt, there is no more interest to accrue. It is a lot of points for very little difficulty.
Role playing wise, ya I get the "in debt" bit. It's a great plot hook. But with so little cash on the line, even the biggest debt is worth only a session or two.
As to the GM adding new negative qualities after the debt is paid off but not bought off with Karma, exactly what new negatives could a GM justify, story-wise? What would fit in place of the moneys paid back? At 30 BP, that is a lot of new disadvantages to call in. A cruel GM could slap physical negative qualities onto the character. A sadistic one could slap a 30BP enemy on him (if that didn't make you taste bile, you weren't thinking about it hard enough).
While it would depend upon what happened at the table, the In Debt quality is not equivalent to any other quality in the book. (See above regarding the 30BP enemy.) I had originally thought it gave you points equal to what BPs normally bought. This is not the case, obviously. However, if the quality did grant that amount of cash, then it would provide a serious advantage to the player in the short term, and be a recurring nightmare for the long term.
What would you, as a GM, replace the quality with, if the PC had paid off the monetary value of the debt but not the Karmic value?