Billing schedule lines can be either terminated completely or line-specific. At the moment, when terminating a specific line with refund, it is neccessary to do double work. The "Terminate" button in the Billing schedule lines section (not action pane header) also just opens the "Mass termination processing" mask. Means, the selection on the specific billing schedule line before does not have any consequence, as you have to remove all lines, which are not supposed to be terminated, afterwards again.
The "Termination" button in the Billing schedule lines section should only be working when a line (or lines) is (are) selected. After that, it should not be neccessary to remove again all the other billing schedule lines, which should not be terminated. For the solution, you only have to enhance the filter criteria on the "Mass termination processing" mask by adding a filter on the lines.