For the future DrinkingHard you can put a system in place that prevents this sort of thing happening. Its a relatively easy thing to do rather than relie on other sites that look at cached history such as the one WG pointed out.
Your system should keep a log of not only the url to send to but the last time the target page was updated along with a page capture of the updated pages of your clients. You can ping for an update by the hour, by the minute or by the day however you like.
I am really surprised you did not have that in place, if you had this would have had never happened and someone would be in jail for credit card fraud already.
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