"Proof positive," he says, still snapping, not caring whether Sirius understand that he means of me being my only self or not.
Somewhat appeased by Sirius's admission that he really did mean for Severus to risk poisoning only himself rather than to create a debt by paying him to heal himself (this is his interpretation), he shrugs again. "If it's convenient to pay in coin, that will serve. If you'd prefer to write a draft, make it out to V. Clayborn."