Uzugami: Bunretsu Zoushoku Ningen

Satoru Takeichi, nicknamed "Ichigo," works at a sawmill. While working, he gets caught up in a mechanical malfunction that amputates the pinky finger on his right hand at the first joint. He searches for the severed portion with the other employees, but they can't find it. However, when he's medically discharged and returns to his apartment, he finds another version of himself there... From then on, a strange shared life with "Nigou" begins.

(Source: Coamix, translated)