Premise: He is a thirty year old who lost his job at a failed company. He dropped a lottery ticket into a Japanese lake and falls in trying to get it and sees a mermaid who saves him. Later his friend won't believe him but jokes he could take a pic and sell it for a ton of money. So he goes to get a picture of the mermaid and finds she is injured, so he helps her. She says she was injured trying to collect coins to give to the shrine by crawling, because if you get enough you can get a wish right? She says she wants to be loved.
Later he comes back, thinking about whether to sell the picture online, and he sees her again. She hears about his money problems, says it sounds fun, and gives him the coins to help him and then fades away. Then he hears the voice of God or something who offers him a deal. (I'd have to reread it, but think it was one year of his life to resurrect her. The voice taunts him: are you content with a boring life?)
He agrees, the coins vanish to bring her back to his relief, and she remarks about having three legs. He realizes she's looking at her her new dick. (Then they live together - cue six volumes.)
https://www.mangaupdates.com/series.html?id=104958 http://sixmanga.me/manga/Ningyo_no_Ouji_sama Oh, someone translated two chapters three years ago. Btw drinking beer turns him back into a mermaid.