From what I remember he's referring to something else plus he won't tell Garam the truth about the condo coz he knows he won't accept it if he hears it
Spoiler
He got mad at Garam's dad for acting carelessly about the whole ordeal thus making Garam cry and worried sick about his mother, especially this close to his exams, which he knows Garam cares a lot about.
Did they fight or yell at each other ???
The dad started yelling coz he said Jaehyuk was being disrespectful towards him
And so Jaehyuk "apologised" to the dad for talking without knowing their personal business but said to his face that him (the dad) continuously saying that he did all of this for Garam's sake when he just made him cry, and created trouble for him this close to exam period, was just annoying. So in the car he (jae) apologized to Garam for potentially putting him in an awkward situation with his dad, but Garam says that the guy doesn't really care about him coz he's actually his stepdad and they never had a close relationship since his mother married him.
And he still apologised to Garam the character development is character developing ╥﹏╥
Yea something bad. Got him a whole new condo that is 10 times bigger than that shoebox he was supposed to live in ╮(￣▽￣)╭