Recently I was told about a person who is a regular customer at a particular store, and for this person it would not be unusual to be given a discount by a friend of theirs who works there. I was asked if this was morally licit. There are some situations and circumstances where a discount may not always be morally licit.
Being given a discount is not morally wrong, but it would be morally prudent to ensure that you have the correct change in order to pay the asked price for any given commodity. In other words: don't intentionally try to short change anybody:
"Render therefore to Caesar the things that are Caesar's, and to God the things that are God's" - Mark 12:17
Permission should always be granted by the propreitor of the business if a discount is being given, otherwise it could constitute as stealing. It would not be okay for a friend working at the checkout to give you a discount if they were not the propreitor themself or with the propreitor's knowledge. If a store assistant offers your a discount, ask if the "boss" is aware of it and is okay with these discounts, and if the answer is 'no' to either, then pay the full amount; don't let them give you a "discount". They may think they are doing you a favour, but what's a couple of dollars when your conscience is being compromised?