
No no no no no. The money was for the ring (thats how i saw it and make sense because how Else could he buy it??) and he didnt want to poisen sangwoo. And the missing pilles where probely because he dropped the bolte and didnt get all of them in it again..
Only how i see this, and i think the author put this scene in to make mistrust so the final blow would be Easier

No. The missing pills is because he dropped the bottle down while trying to discreetly hide the ring he bought with the money he borrowed. But Sangwoo at that time is already full of insecurity that Bum is trying to leave him (after having a heated conversation with Bum's friend) so he thought Bum is trying to poison him and he puked. Because he thought of him as his mother who tries to kill him.
Did Bum really wanted to kill sangwoo ? I didn't understand the part with these pills
And he borrowed money to buy what ?