The purpose of obtaining consent, also referred to as 'informed consent' is to enable a patient to make an informed decision about whether or not to undergo a procedure. Common sense would indicate that if you don't obtain consent until after a procedure has already taken place, then you really haven't obtained consent at all, just by definition, since you would not be able to show that the patient had the opportunity to make that decision. There would be no purpose to obtaining retroactive consent other than to obtain paperwork which likely would not hold up in a court of law in any case if the patient did not sign it until after the procedure.
There are situations where consent is not required, such as when the patient is unable to give consent due to competency or an emergent situation, and other situations where documentation of verbal consent is sufficient. It's conceivably possible that you could have a patient sign an attestation that they gave the provider their verbal consent prior to the procedure, but I can't imagine anyone would consider a retroactive consent to be legally valid.
This is kind of a complex legal and medical-ethical area you are asking about, and the laws about this could vary from place to place. You may wish to speak to an attorney in your state and explain the specifics of the situation you are encountering to get a clear answer on this. I don't think you'll find an easy answer with a clear source of documentation saying when this may or may not be acceptable, but just on the surface it certainly sounds inappropriate to me.