The only return to sender spells I know send something sent to me, back to them. They are not used to just give negativity back to someone. They refuse delivery of a spell or other directed energy back to the sender. That isn't want you are doing here. You are trying a Karmic return spell. You want his Karmic actions returned to him. This is also known as Karmic balancing. When you call for balancing of someone, the price is YOU will also be balanced, and have to pay for negative Karma of your own. I speak from experience. I did a balancing on someone once. It worked on them, and many people saw them for what they were, and cut them out of their lives. The price was one of the hardest years of my life, with one Karmic debt after another coming due and demanding to be paid. Be very careful with what you are planning. If you are sure you are willing to pay that high a price, you are free to do so. Stay safe out there.