I was at Nationals Park on Saturday nin the happy hour section and ordered two Nutrl drinks. The vendor held out the card reader, I tapped my card, and then he immediately took the reader back before I could review or close out the transaction.  
It felt off, so I asked for it back. When he handed it over, I saw he had added a $2.50 tip for himself without my approval. I took a picture of the reader. I’m usually a $1 per drink person when crack a can at Nats park. He added 25%, which is one of the highest preset tip options.  
When I told him I was going to Guest Services, he tried offering me free beer, which made it feel even sketchier and I felt validated he did something wrong.  
I went to Guest Services right away, missed probably two or three innings waiting to talk to managers, and they said they’d refund me. A manager told me there were interviewing the food vendor and he was saying he does not speak Engliush well and that it was all a big misunderstanding. I questioned the manager asking why the vendor never let me close out my transaction by adding my own tip. 

I also told the manager this was basically fraud and that I hope the vendor gets reprimanded. And I know it's a fools errand but recommended the manager audit the food vendor's other transactions.

In the end, I felt like Guest Services wasn't taking this serious.

Has anyone else had this happen at Nats Park before?

Beware: Vendor Added His Own Tip in the Happy Hour Section