That actually makes no sense.
If your brother handed you cash and said BUY IT FOR ME that would be construed as a straw purchase, never heard of that for ammo purchase anyway. You just asked to "Borrow money" nothing more. I have gone into a liquor store at the check out I was short a buck, asked my friend who was with me and wasn't buying anything, handed me the buck and no questions were asked.
I am sorry but the clerk had no clue what he was talking about. Should have asked for the manager. Was it the Wally world on 120th and sheridan or the one on 136th and Huron?

