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NVM, I found it. For those who are interesting in reading it, the manga is called Yurusareta Uta by Naono Bohra. One shot and enjoyable. It's a sweet love story between a demon and his master.
got a follow up story ichido dake no uta
There is a manga with similar theme, but it's with a gun merchant who bought a demon from the demon market, who refused to sing or fight. But he used his power to defend the gun merchant. Can someone please help me find that manga? The demon looks almost like Ian from Mori no Koe