beta
I think the reason he left the money behind is to appear more scary in the eyes of the bad guy. It kinda shows that he's willing to do anything for Seo (that includes beating the shit out of that dude since he fucked around and found out). Mr. Kwon is rich af, that money probably is a small amount for what he has, and if Seo is offering to pay it back, he'll have an excuse to keep him close.
A lot of people are commenting the "issue" of him leaving 1Billion won behind, but the mf of Mr. Kwon is willing to tear down his grandfather's business for shits and giggles (not really but you know what I mean). He is so rich that money means nothing to him.