At Financial Cryptography 2003, Jakobsson, Hubaux, and Buttyan suggested a lightweight micro-payment scheme aimed at encouraging routing collaboration in asymmetric multi-hop cellular networks. We will show in this paper that this scheme suffers from some weaknesses. Firstly, we will describe an attack which enables two adversaries in the same cell to communicate freely without being challenged by the operator center. We will put forward a solution to fix this protocol. Then we will describe another method that allows an attacker to determine the secret keys of the other users. This attack thwarts the micro-payment scheme's purpose because an attacker can thus communicate without being charged. Finally we will suggest some solutions to counteract this attack.