My Dear Enemy (2008) on Netflix

Jobless, single and in her early thirties, Hee-soo is miserable. Desperate, she sets out to find her ex-boyfriend, Byoung-woon, who owes her $3,500. Rather inconveniently, it turns out that Byoung-woon is also penniless, but he cheerfully claims he can get the money from a series of girlfriends. Suspicious, Hee-soo decides to trail Byoung-woon as he makes the rounds of his varied lady chums, just in case he tries to deceive her. Thus the pair embark on a strange, nostalgic journey together.