It is possible, in some occasions, when those out of good intention but under unjust means, obtain benefit for us and we take it justly. On the rest of the other occasions, we do not get ourselves involve at all whatsoever.
But when we take it, we remind ourselves not to follow their injustice. Their good intention is annulled.
It is just, for us, because what's done is done and there's no need to leave things to waste. It is crucial also that we do not hold dear to those benefits which are procured by others unjustly, despite their significant values held dearly by others. What we have received we keep them responsibly.
Are we to be grateful to them? Yes. Do we owe them anything? No, we shouldn't, nor do we need to make up our own repayment to them. How is this grateful them? It is not what they do that we are grateful, but what we receive that we are grateful of.
If there is a need to return a favor, there is no need to give one, for there is no favor owed. There was no lesson learned from them but from ourselves, that we be less self-involved. If we help anyone, it is based on our better judgment, not on any favor that was based on unjust means.