Seems weird that a promotion aimed at, I'd say, 50-50 at doing something nice...and looking like you're doing something nice, would prompt an employee to go all "vet police". Who cares about the back story? Worst case scenario, some dude gets a free burger...what have you gained by questioning his word? Best practice- if he says so, it's so as far as you're concerned. Step back from the power trip Mr. "6 months ago I want making minimum wage".