The subjective considerations of an apparent agency definitely render it the most difficult to ascer

The subjective considerations of an apparent agency definitely render it the most difficult to ascertain. If I buy a soda from a clerk at the store, and he offers me 10% off, I assume he has authority to offer the discount. But, if it turns out that clerk was not an employee, but a random guy off the street, will I be in trouble for taking the 10% discount? What factors may weigh in?