Registrations that have an additional donation made towards them are "linked" together in Sphere.  While it is possible to reverse an Additional Donation but not the Registration Fee, it is not possible to reverse the Registration Fee and keep the Additional Donation.  This is because the Registration is the main component of this transaction, since it was what originally allowed this supporter to register for the event.

In order for the participant to still keep their Additional Donation towards the event, they would either have to stay registered for the event, or the entire transaction can be reversed and the participant can then come back to the event site and make an individual donation.

Note: Due to the way Sphere handles a Registration with Additional Donation attached, the reversing of only the Registration Fee cannot be done through our data team.  There is a risk of data corruption when attempting to perform this type of action, so reversing the entire transaction and having the participant make an individual donation is the only way to resolve this issue.