H1B Withdrawal process for employer once employee resigns

I am moving from my employer A to employer B based on receipt notice. I have asked my current employer A if he can hold the H1B withdrawal process until the H1B transfer completes, but my employer is slightly reluctant on it and saying as per USCIS we need to revoke. But I know cases in past where they have not revoked for few others, though they left.

In this regard, i am trying to understand if there is any hard rule imposed by USCIS for employer A to withdraw H1B of resigned employees within 30 days ?

Appreciate your help on this.

No, there is no rule. I suggest not asking too many questions to employer A about revocation.