We have dealt with Ballerina for over 25 years and have never had a shady dealing with them. I am inclined to believe ballerina as their has been a resent wave of counterfeit 100 bills being distributed and used on SXM. Some even given out by a Bank on SXM. Merchants use a specific pen to detect counterfeits. I presume ballerina checked these after you left instead of in front of you to save time to get the reset done.
Counterfeits are hard to detect with the naked eye is why they use that pen.
Sorry it happened to you but I think Ballerina is in the clear on this one.