With the company I work for if I have passengers saying the TVM is not working I will ask for proof like a photo with date it was taken. If they cannot show this to me I will then call someone in our company that can live check the TVMs and tell me if there is an issue.
We also get every hour an automated email telling us which machines are our of order.
This is not 100% as if the screen does not work and the touch screen is misaligned (Press on a letter and it registered you touched at another area) this cannot be reported by the machine as it thinks it is still working. When I find these I email the person in charge of TVMs and also an email to my Revenue colleagues that the machine is down for now (If no booking office and the TVM is the only way to purchase a ticket).
Not sure that this method is 100% foolproof for a number of reasons. Obviously, not everyone has a smartphone to photograph or film the non-working TVM. Trying for a third time to buy a ticket, just for the purposes of filming or photographing it, isn’t going to go down well with the queue of people building up behind you, especially when the train is due! The card payment not going through could be a fault with the TVM, in which case the machine may report it. It could equally be a fault with the physical card, the communication network between the TVM and the card issuer, or the card issuer’s system, in which case the TVM may well think that all is well.
My solution is to always carry a reasonable amount of emergency cash, say £100, in case card payment systems throw a wobbly. Enough to buy a train ticket, tank of petrol, cheap hotel room, taxi fare home, bag of groceries, etc. (Maybe a prompt for a “how much emergency cash do you carry” poll?)
In the OP’s case, trying to settle a Promise to Pay using a card AND admitting to not having the cash to do so, has raised suspicions that they were attempting a “pay when challenged” scam.