technically speaking, you neither need the permission of your employer, nor do you need to inform them, as in i485, you are the applicant (for i140, the employer is the applicant, you are the beneficiary), as long as you can provide the minimum necessary documentation.
HOWEVER, practically speaking, it would be extremely difficult, if not impossible, to keep an i485 going without the knowledge and consent of your employer. there would be times when you would need their support: at the time of filing, you need your employment letter, and later, at RFE time, when you would need to get a current letter from them, and possibly other documents. even if you got away with filing for right now without their knowledge, i don't know if you want to be in the situation of springing the surprise on them later when they would need to know.
please understand that i am not at all saying it is impossible; you might even get away with it, who knows. but if i were you, i would try to my best to bring them around; a compromise could be that they file your i140 through their means, and you file your i485 through your lawyer and bear the expenses, but with their knowledge and consent. is money the issue? if it is just that they are not too sure about the procedure, a lawyer could definitely help you out.
i would DEFINITELY discuss the pros and cons of this with a lawyer, and get his/her take on it.
CAVEAT:
i am neither a lawyer or immigration expert; the above is not legal advice or recommendation. DO consult a qualified immigration attorney before deciding anything.