I recently had a similar situation at the Gare du Nord station in Paris. We were trying to purchase tickets for the train when this guy comes along and forcefully takes over the machine saying it doesn't take credit cards (which we knew it did because we've used them before). He changed the language back to French and proceeded to buy the cheapest metro ticket available then asked us to give him cash based on the more expensive distance we actually wanted to go. Luckily, we didn't have any cash, he even offered to accept pounds or dollars! Eventually after skulking off we went back to the machine and worked it out. If we had taken his tickets we likely would have been picked up by one of the frequent ticket inspections and given a hefty fine.