Zhang Heng and Scarlet agreed to meet at the checkpoint where both their identities would be revealed. It was a safe zone, so neither had to worry about being ambushed.
Zhang Heng used his Oath Ring to authenticate Scarlet's words, then exchanged the upper part of the Mikazuki Munechika for Scarlet's scabbard.
Scarlet checked the hilt she received to confirm that it was genuine before slipping it into its satchel. She then instructed Zhang Heng on how to use the scabbard. "Cut the flesh of your palm and let your blood drip onto it, about two milliliters. After three minutes, you will find yourself in the special quest."
The transaction was complete, at this point, and eager to reforge her Mikazuki Munechika, Scarlet didn't linger around the checkpoint, leaving hastily after a handshake with Zhang Heng.