Please don't sandpaper off the oxidised green copper from the rivets. They will scratch and look that way for ever after.
Carefully polish off with Brasso or some other metal polish. Don't rub the blackened rag on the leather, though.
You may need to mask off the rivets with a not-too-sticky tape (such as for masking skirting boards when painting walls), and polish up the rivets before using Brooks proofide. After that, your shorts will auto-polish for you.
If you dont polish up the rivets, you will get green shorts..