I believe what Lim said is correct, 2 months of pay is already a form of compensation say if you choose to cease work immediately. So in other words, it is also a part of your right.

In the meantime, your offer letter also didn't state the boss has the right to restrain you from resigning with immediate effect, of course it's his right to say anything but it's a non legally binding right. So no big deal, just push through your resignation.

good luck!!!