It seems as if xeno is just itching for a reason to complain, when it would have been easier for everyone involved if he just had cash on hand. The vendor obviously isn't forcing him to charge the transaction, or even preferring it. Otherwise they'd have the dongle already attached to their iPhone or iPad, have the app running, and likely have been using it without incident (gasp! Even sending SMS/email reciepts!) many times that day and before.
I have a Square dongle, and a GoPayment one. I've used the Square to clear the remaining oddball balance from a Visa gift card into my checking account, and it was easy as "Open app, plug dongle. Enter amount, swipe card, sign something, click "Continue to receipt". Type a phone number or email, click Submit (or skip receipt)." That's it.
In fact, I just charged myself $1 from another bank's card...which will cost me $0.03 (no big deal). It took not even 30 seconds from when I unlocked my phone, to getting the confirmation text. For someone who has used it before to run transactions, or even many times as a small vendor should have, it's not that complicated!