Martin Gantt received $600 in advance for tutoring fees when he agreed to help Josh Smith with

Question:

Martin Gantt received $600 in advance for tutoring fees when he agreed to help Josh Smith with his introductory accounting course. Upon receiving the cash, Martin mentioned that he would have to record the transaction as a liability on his books. Smith asked, “Why a liability? You don’t owe me any money, do you?”
Required
Respond to Smith’s question regarding Gantt’s liability.