אתי דיבור ומבטל דיבור

Review the תוס׳
Question of הבה״ח אבא יוסף גולדשטיין - This case you are trying to be מבטל the תרומה which was taken off by the שליח, that was not your דיבור, even acc. to ר״י we only find that one can be מבטל his own דיבור we never find that you can use your דיבור to be מבטל someone else's דיבור ?
Answer of הבה״ח יהודה הלוי לוין - You are not being מבטל the תרומה but rather the שליחות
Question- How can you be מבטל the שליחות if the שליח already did his job, he was מקיים his שליחות , what are you being מבטל ?
Learn the רש״ש
Is the רש״ש saying a different answer than תוס׳ ?
Learn the פנ״י
Learn the רשב״א