Those "Black Eyeballs" are actually creatures from the Exotic Realm, results of an accidental parasitism within human bodies.
Each glass bead contains the Extraordinary Power possessed by the creature, and if consumed by humans, the consequences are unknown; one might become healthier, die directly, or undergo radical personality changes. In any case, they represent something chaotic and dark.
However, to the Night Beast, they are extremely delicious Food.
The Night Beast desired to eat them, and Tang Qi could sense it.
Thus, Tang Qi told the Night Beast that his master was too poor to afford them.
The little guy from the Exotic Realm, inexperienced in worldly ways, clearly did not understand the treacherous nature of human hearts. It only knew that those Black Eyeballs were rare and delicious foods, and that it was quite normal for them to be hard to obtain. So after a round of "haggling", the Night Beast agreed to modify some terms of the Contract.
