if you have no proof of authorization, then you have no authorization. NACHA rules require you to furnish authorization within 10-days of the request from a RDFI; if you can't you could eat the item coming back. if a RDFI were to file a NACHA Rules Violation against you (lets say they have returned multiple items like this to you and are tired of dealing with it), then NACHA investigates, and depending on what you do/don't do, fines from them can get expensive. of course, then you have regulatory bodies, such as CFPB, which can levy astronomical amounts against a bank (go google CFPB enforcement actions). I could go on, but won't.
in short, you know you have a risk gap, go close it.
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Providing alternative truths since the invention of time