형이다마는 in this sentence is wrong, right?

Question

I am learning the ㄴ/는다마는 structure, and i have a sentence as follows:

그 사람이 내 형이다마는 서로 얼굴을 안 본 지 오래되었어요. (That person is my brother, but we haven't seen each other for a long time)

As far as I understand this structure, ㄴ/는다마는 = ㄴ/는다고 하다 + 마는. Also from what I've learned about indirect quotes, "Noun + 이다" must be combined with 라고, not 다고. Therefore, the above sentence should have been rewritten as "그 사람이 내 형이라마는 서로 얼굴을 안 본 지 오래되었어요", right?

Answer 1

That sentence is right.

• 형이다마는(O) 형이다지만(O or X)
• 형이라마는(X) 형이라지만(O)

(O)형이다마는/형이다지만은 : 형이다("라" is also possible) 고 하지만은

(O)형이라지만은 : 형이라고 하지만은

(X)형이라마는 : We do not use such abbreviations.