Just a guess but I feel like Hae Seo threatened Taekyung's foster mother with knowing her killing his own mother. If she wouldn't comply to his commands he would tell the police everything he knows or smth like that. Foster mother seems like she is kind of rich so it's no wonder where Hae Seo acquired that much of money in a short time.
Potential Spoiler
Just a guess but I feel like Hae Seo threatened Taekyung's foster mother with knowing her killing his own mother. If she wouldn't comply to his commands he would tell the police everything he knows or smth like that. Foster mother seems like she is kind of rich so it's no wonder where Hae Seo acquired that much of money in a short time.