If you accidentally over refund someone (meaning you refunded them too much), follow these steps:
Step 1 - Adjust Credit Amount
Change the credit amount from the amount over-refunded to the amount you should have refunded. For example, say you refunded someone $100, and it should have been $50. Change the credit (amount in blue) from $100 to $50.
Step 2 - Post Payment for Difference
Next, post a payment on their account for the difference in what you refunded and what you should have refunded. For example, you refunded $100, but it should have been $50. You changed the credit to $50. Now you post a payment to the customer's account for $50 (the difference).
In case that doesn't make sense, here's an example:
The customer paid $300
You refunded $100 ($50 more)
They received back $100
You credit them $50, which is the amount you should have refunded them
Post a payment to their payment method for $50, which is what they owe you back from the $100 refund