Funny thing, that's one of the few things from Excalibur i actualy keep thinking about. Mainly what twist could be done with repaying her debt.
After all the thing she broke was "priceless". That opens up various ways one could argue her out of that situation.
Like for example finding something so utterly worthless that it counts as valid exchange for the thing she broke, simply because it has so little worth no price can seriously be applied to it, as there is no demand for it, making it "priceless" too. Or counter arguing that if something is pricesless it has no actual worth as item to purchase, since no price means nobody can ever buy it, meaning it should have never been on the market, therefor her destruction of it does not warrant keeping her in captivity.
Or arguing that Red Root has a worth and one can apply a price to her freedom, meaning she would have never been valid to be taken as exchange for the item.
Etc. etc.