The medicine given to the first slave starts affecting the surrounding slaves due to the effect: [Range Expansion (small)].
While the medicine was cheap, it was still able to heal the slaves since their illnesses were only superficial.
Slave 1: “….”
Slave 2: “No way, it’s a miracle!”
The medicine’s effect seems to have worked, as the slaves' skin color gradually returns.
Slave Dealer: “As expected of Hero of the Shield… giving strength to sick slaves at a bargain price instead of paying more for healthy ones!”