Yami Baito-saki wa Isekai deshita

Since his mother collapsed, Ryuji Hashiba, a high school student, has had to give up his dreams and work to earn the money to pay her hospital expenses. One day, Ryuji finds an unbelievably lucrative part-time job, and even though he knows it's shady, he participates in order to pursue his dreams again. When he follows his employer's instructions and passes through a mysterious door, an unknown world stretches out before him, and he is suddenly attacked by monsters...!?

(Source: Kadokawa, translated)