If your customer has a receipt for the 70% refund, that portion actually does fall under Reg E. 1005.11(a)(ii) An incorrect electronic fund transfer TO or from the consumer's account.
You're correct about the remaining 30% not covered by Reg E, but under Visa/MC you may be required to assist. Depending on the amount, it may be simpler (and cheaper) to refund the remaining money.
If the merchant has issued a credit slip to be refunded within 5 business days, you don't truly have a dispute until that time has expired. Almost like a dispute that is issued on a preauthorization swipe - the transaction hasn't posted yet, so there is no dispute until it has. After 5 days, if the credit has not appeared, file your dispute for credit not processed, then file for the remaining 30% as merchandise not received.
If the credit does process, you can still file the dispute for the remaining 30%
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