How do I take money back out of a customer's wallet if I credited too much?

Use a manual debit:

1. Go to Plugins → Wallet → Customer Wallets and click Manage on the customer.
2. Under Manual adjustment, choose Debit.
3. Enter the Amount and a reason. The reason is required and is kept in the history.
4. Click Apply.

You cannot take out more than the current balance; Wcart shows "Insufficient balance to debit". A frozen wallet cannot be debited either, so unfreeze it first. Then add the credit to the right customer the same way with Credit.