Usage of the word 欠 and 了 in 那我一分钱也不欠你的了
Sometimes it is hard to guess without looking at the full context.  However, one of the possibility of having 了 is to represent "I owed but no longer is".  "了" in many cases give a hint of something happens just now, which is different from the past.  

e.g.

我不愛你了 vs 我不愛你

我不愛你了 actually means I used to love you, but now I don't love you anymore.
我不愛你 simply means I do not love you.

Similarly

我敢吃榴槤了 vs 我敢吃榴槤

我敢吃榴槤了 means I was not dare to eat durian but now I am.
我敢吃榴槤 simply means I am dare to eat durian.