If a cashier or other low level employee makes a financial decision in your favor ("keep the change" or "I'll let you use that coupon even if it expired" etc.), without checking with a manager/owner, may one rely on that employee, or is it considered stealing? Does it matter if the store is owned by a Jew or a gentile? Please cite sources.
"The hand of the employee is like the hand of the employer" (Bava Metzia 10a.)
I would say that, unless you have a reason to believe that the cashier is trying to his cheat his employer by giving you a special deal, you can rely on the cashier's decision.
Here is an interesting shiur that gives more detail on this concept, and practical applications thereof: