Responses
I think it's because he took the bus after she dropped him off, Bum made her take him to the wrong address and then took the bus to Sangwoo's house who saw everything from where he was, "you know you can see pretty far from up here" and then Bum repeatedly says he took the bus, so I'm guessing he asked for bus money
he wanted to scare her off with this request. he didn't want to meet again, she was too pushy.

Why everyone just forget about Bum asked the girl to lend him money! But why?
I didn’t get it.