There is no built in way to do this. The following steps work as a work around in most scenarios.
  1. Use these steps to to do a partial offline refund (otherwise known as a partial deletion of the transaction).
  2. Create an offline gift for the same amount as the partial offline refund in step 2.
As long as the new transactions total to the same amount as the original reporting should line up in Luminate Online. You may need to communicate with your accounting team depending on how they do reconciliation.