Hi There!
It can't be both ways--the WSUD is a statement from the consumer that is unequivocally stating the transaction is unauthorized. If it is authorized, the form should not have been filled out.
Assuming that you are certain that it is authorized or that your receiver confirmed it is authorized--R10 is a no-go.
Did the consumer try to resolve this with the utility yet? If this is indeed authorized activity, the consumer's first step should be to contact the utility and ask them to locate the payment. If the utility then needs proof to track this, the ACH detail record in conjunction with an account statement or confirmation on bank letterhead that the transaction posted should be more than enough for the utility to locate the transaction and credit it properly on their end.
The only way R10 *may* apply is if this was done through a third party bill payment service rather than directly through the utility. If the third party bill payment service debited your receiver but did not transmit a corresponding credit to pay the utility, this is called an Incomplete Transaction and you may now use R10 in that scenario. This is a newer rule in the NACHA Operating Rules. I would however point out that the rule specifically states "....a partial or erroneous payment to the intended third party payee is not an Incomplete Transaction." There are a lot of requirements to using R10 for an Incomplete Transaction, I would ensure you meet them all before using it.
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Nothing I say should be considered legal advice or the opinion of my employer.