Ch 51 - 54 of the webtoon = around chapter 41 (as ch51, Yoohan sleeping in front of Jay's door) - 42 of the novel (middle ch54, in the elevator "are you rich ?", both of them in the subway and the confrontation for the info leak). But the sex part (ch52-53) was not in the novel.
https://www.novelupdates.com/series/payback/
Review novel : https://t-n-c.tistory.com/14
https://m.blog.naver.com/jinne7/221824049026
https://sirococo.tistory.com/490
Raws novel : https://m.blog.daum.net/banana917411/8
Raws : https://www.bomtoon.com/comic/ep_list/PAYBACK
newtoki301.com/webtoon/17292807/페이백-PAYBACK?stx=페이백
https://toonkor218.com/%ED%8E%98%EC%9D%B4%EB%B0%B1-PAYBACK
https://vinload.com/manga/beahcc-payback/
https://mintmanga.live/vozmezdie__A533b (en russe : Возмездие )
https://zinchanmanga.net/manga/payback?id=385
https://mangabuddy.com/payback
Bad trad 89 let's wait until 88 at least
Payback