Bynum would only cost the Cavs $12 million if he is on their roster on 1/7. Only $6 million of his salary is guaranteed. The Lakers aren't looking to sign Andrew to a new contract. If the trade were to be completed, they'd cut him by Monday, thereby saving themselves his guaranteed money and Pau's salary. The Lakers would be under the cap after this move.
However, they are going to be under the cap anyway after this season and after letting Pau walk so there isn't any urgency to make this deal. That's why it likely won't happen because if it does, it would have to be on the Lakers' terms.