Is seller ( owner ) financing is ILLEGAL with existing mortgage ?

[Tan Mehedi]

Hey community, Is owner financing is illegal with a existing mortgage ? Example , if a house worth say $200k and seller has mortgage balance remaining for $50k . is it illegal to sell that property with seller financing ? i know it can be done through "subject to" or wrap around mortgage but IS IT ILLEGAL ? because one seller says he talked to a attorney and he said its illegal. i wanna get clear on that. This is help all new investors.