You can write USCIS immediately and withdraw your I-864 and any support of her application. I would do It ASAP.
You can also go to your local USCIS office via Infopass to withdraw the application. I would send any notice visa certified mail or with some sort of confirmation
you do not have to be divorced to withdraw your I-864.
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Neil I Fleischer
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There is no way to revoke an I-864, but you aren't liable on it if it isn't ultimately used in support of approval of the I-485 Adjustment of Status case. You may want to send USCIS a letter explaining the situation, and including copies of the divorce filings and the Receipt Notice for the I-485.
If the I-485 is ultimately approved, you are liable on it - even after divorce (See the legal guide link below, items 6-7, for where you are and aren't obligated).
If this case gets approved, she will get "Conditional Permanent Residence" - a two-year green card with a requirement that she file shortly before the end of the two years to remove conditions and get a full ten-year green card. This is usually a joint filing with the US citizen spouse - once divorced, she would need to claim either that the marriage was bona fide (tough to do when you broke up so soon after she entered, though possible) or that you were abusive.
Since you are already divorced and you are no longer the petitioner on Form I-864 you should be ok. But as a precautionary measure i would withdraw Form I-485 and Form I-864 by writing to USCIS or by going to your local USCIS office through an Infopass appointment.
Maud Poudat, Esquire
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In addition to sending a letter, via certified mail, to USCIS notifying them that you're withdrawing your support of the I-485, I would follow-up with an Infopass appointment to make sure it's in the file. In the letter, you should state the reason for your withdrawal and dissolution of the marriage if applicable. Furthermore, depending on where you are in the divorce process, and the grounds for divorce, it may be helpful for your family lawyer to speak with an immigration lawyer to determine if the divorce proceedings would substantiate your basis for withdrawing the I-485 and possibly obtain an annulment, or help your spouse obtain residence status base spousal abuse.