Let's say I go to a store in the morning and buy something that costs exactly $1 (includes tax). I give them a $1 bill and I'm on my way.
Now, let's say I go back to that store later that same afternoon and buy something that costs exactly $4. I give them a $5 bill and get a $1 bill as change. Assume this $1 bill is the exact same bill I made my purchase with this morning (assume I recorded its serial number).
Did I actually spend that $1 bill this morning, considering I just got that same bill back this afternoon?