For a partial refund the Reverse option cannot be used, as Reversing an entry will always offset the full amount of the original donation. If the full amount was refunded, please see How do I refund or reverse a donation? for steps on reversing the transaction.

Instead, a Disbursement can be added to the donor's account. Adding a Disbursement will allow you to customize the negative amount to apply to the donor's account, so you can accurately track the amount refunded:
  1. Open the Constituent account
  2. Click Journal in the header
  3. Click the Add... button
  4. Click Disbursement
  5. On the New Disbursement screen, fill in the amount that was refunded in the Disbursed Amount field on the left hand side
  6. Select the same Fund, Campaign and Approach that was selected on the original transaction
  7. Select the appropriate Gift Type
  8. Click Save And Go To Journal