According to EFT Mastercard rules, if the merchant had told the cardholder that they will be giving the cardholder credit, we can not send in a dispute for 30 days. This gives the merchant enough time to provide the cardholder the credit that they promised. If the customer does not receive the credit in 30 days, then a dispute can be submitted.

How does this affect Reg E rules?

Regulation E §205.11(a)(1)(iii) includes an EFT that's missing from a periodic statement as an error. So, if there's been a statement on the account since the merchant promised to provide a credit, the customer can assert a claim if that credit isn't there.

Until the 30-day mark required by MasterCard, you are pretty limited on how you can proceed. One option is to attempt to contact the merchant to ask about the promised credit, after checking to ensure it hasn't been posted since the statement date. That might get things moving without having to file a dispute.