Su Yang returned home with the porcelain teapot, which was actually not a cauldron, but a magic artifact, and moreover, it was a Ghost Cultivator's magic artifact.
A short while ago, angered by that sleazy man's attempt to raise the price on the spot, Su Yang had covertly opened this magic artifact. When the box opened, the shrill sound was the voice of a ghost spirit, and the black mist was resentment that toppled the shopkeeper. Su Yang slapped the shopkeeper twice and took away the resentment, and naturally, the shopkeeper was fine.
Although this wasn't a cauldron, the resentment of the ghost spirits inside could be transformed into Spiritual Energy. Furthermore, the material of this magic artifact was not bad, so Su Yang could remodel it into a cauldron.