From what I could gather from reading this and previous chapters
Most of the Fu city officials are already on Liu Bei side and would happily betray Liu Zhang when needed, and the army of Gao and Yang are already leaderless. Therefore, Zhang Ren propose that they would give the "side dishes" that refer to the officials and the armies if Pang Tong hold back from feasting on Zhang Ren and the rest of Liu Zhang supporters plus Liu Zhang himself. Therefore, obtaining the greatest long-term benefit since assassination there while having the chance to succeed are also gonna damage Liu Bei reputation in the long run and might plant seeds of rebellion for people in Yi province.