You cannot partially refund memberships that have contributed membership revenue.

To change the membership program so it does not have contributed revenue, follow the steps below:
  1. Go to Memberships > click on your membership program.
  2. Go to the Rules Tab.
  3. Click Edit.
  4. Mark the option No, the entire amount is earned membership revenue.
  5. Click Save.
For all memberships sold after this change is made, you can partially refund them.  For memberships that were sold while the Rule said that the tax deductible portion is contributed revenue, you cannot partially refund those.