Leaving a Flat outside the Contract Date

Hi, I have read the Dummies Thread but am not sure about this one.

If I wish to leave the Flat I rent before the 3 Months Date, I understand, same as in Germany, if I find a "suitable" Person wishing to take over the flat then the owner has to accept it?

Question is, what is confusing for me, on some places I read that when I send the owner (WinCasa) the "leaving" Letter I need to send already the suggested person who wishes to sign a new contract, but this does not make sense to me.

Can I not send the Letter, ask their permission to already advertise the Flat and then look for someone who wishes to rent from X Date?