Answer:
I would act with utmost honesty & integrity, & return back the extra change mistakenly received by me.
Step-by-step explanation:
I think the entire scenario is a matter of honesty & integrity.
If I had mistakenly taken change of $20 against having given $5 at a fas food drive. I would try contacting the 'fast food drive through restaurant' & communicate them that I have mistakenly got extra money change. Thereafter, I would pay them back the balance money by online transferring it, or while going to office the next day.
If their contact details are not being found, or are not approachable : I would directly opt for the 2nd option, ie return them the change while going back to work the next day.