Maybe the guy in the line-up was upset with the cashier wasting his time, rather than the woman returning the milk? 
I mean who cares why she was returning it, it's a few bucks and it's refundable, just give her the cash back. Probably exactly why the other employee said "just do it". 

Obviously I wasn't there, but that scenario immediately came to my mind while reading your story. Why hold up an entire line of people just to find out the reason someone returns a few litres of milk, makes zero sense. 

I hate to say it, but you may have been "making assumptions" about him.  Maybe he had a bad day, was in a hurry, and was impatient with the cashier, rather than racist? 

Saying that, I get where you're going with the rest of the article, and agree.