A vendor charged our card too much for a payable and has now issued a refund. How do I enter this?

I've got just the steps you're looking for, @jeanmarie.

The best way to handle this is by recording the full amount of the overcharged Bill or Expense and then create a Deposit to account for the refund. I'll guide you through doing this below: 

Recording a Deposit for a Refund

1. Begin by ensuring the Bill was entered for the full amount you were (over) charged for. This way the Bill Payment can be matched to the Expense transaction that comes over from your bank.
2. Click the Plus (+) icon and select Bank Deposit. Select your bank account at the top.
3. Under Add funds to this deposit, select the exact same Expense account you used on the Bill/Expense. Enter the Amount that was refunded to you.
4. Click Save and close.

Check out this detailed guide on recording bank deposits for additional information. I'm attaching a video below which demonstrates the creation of a Bank Deposit for a Refund:

Doing this will reverse the extra expense incurred from the overcharged transaction while allowing you to reconcile the actual cash flow through your bank account. Please keep in touch with he here should you have any additional questions or concerns, I'm always available to talk QuickBooks. Thanks for reaching out and take care.