Assuming that your Sum field already is a summary field, you'll need an additional calculation field with GetSummary() to get the summary field's sub-group value for a sorted found set; like
500 - GetSummary ( sSumOfDonations ; LastName )
where sSumOfDonations is the summary field (Total of), and LastName the field the sub-summary part is sorted by (the two arguments for GetSummary() ).
Correct answer except for one problem. I have a couple of donors with the same last name. They got lumped together under the last name. If I sort by last name and then first name they still get combined. I had to use FirstName in the GetSummary, then have a Subsummary part by LastName followed by a Subsummary part by FirstName. The fields for LastName, FirstName, and VolumeAvailable are in the Subsummary by FirstName. I made the Subsummary by LastName part very narrow so it just looks like a line between donors which is convenient. Thanks for the help.
Donors should have unique ids -- use a serial number.