That's ridiculous advice. OP has proven disloyalty and/or lack of communication by accepting the other job. That easily qualifies as grounds to renege the offer, had the offer been offically made. If OP had actually spoken to the boss about this opportunity before accepting the new job, better advice would be this: Contact the current company's higher-ups to see if it was a legit offer. If it was, OP could explain his financial motivation, and perhap cite difficulty with the current boss as another motivation to go elsewhere. (And truly, I'd consider it a factor if the boss was in charge of nominating promotions and had thus far neglected the OP). Maybe they'd consider keeping him if they thought the problem was an individual relationship.

Not true. A lot of food service places recruit for management in-house. They may have planned to train him first for the salaried position. Some GM programs take months of specialized classes and "internships", ie working an AM position, within the company. The boss could've meant [s]he was going to enroll OP in the right program to start the journey toward a GM position, rather than offering him an immediately open one.