Peddler in Another World: I Can Go Back to My World Whenever I Want! Volume 10 by Hiiro Shimotsuki

All in all, Shiro’s excursion to the city-state of Orvil was a roaring success. He freed Kilpha from an unwanted engagement as well as from her obligations to her tribe, liberated the beastfolk of the Dura Forest from servitude while also rescuing them from the twin threats of starvation and disease, and thwarted the corrupt prime minister’s plans to overthrow the rightful king. And that, as they say, is that. But is it? On his return to Ninoritch, Shiro finds out that the collars used by the prime minister to subjugate various powerful monsters were crafted by demons. Recalling rumors of shady merchants peddling forbidden items all across the continent, he grabs his winter boots and warmest jacket, as there is only one way to find out the truth behind these potentially linked mysteries: a trip northward to the isle of the demons. Celes reluctantly agrees to take Shiro and his companions to her homeland, but can they put a stop to this unchecked dangerous trade?