Thank you kindly.
The gemorah in Chulin (131a) Brings down the concept of “Mishtarshi Ley”, which simplisticly means that you are gaining from the other persons loss. Obviously this is not so simple (as Tos. there points out) as for eating Matnas Kehuna you are Patur, but giving it away as a substitute for your own item you are Chayiv.
The question is twofold:
1: What are the halachic gedarim of Mishtarshi
2: Are there any modern poskim who bring Mishtarshi in teshuvos as a reason to be chayiv, and in what sort of examples.