In case of I-539 is in pending and i-94 already expired, Will My Mother allow in USA back if she leave before I-539 change of status? Please help.

If your mother leaves by the date that you have requested on the extension request, she should be treated like any other visitor to the US when she attempts to reenter at a later date. She does not need to leave the country before the expiration of the I-94 so long as she has filed a timely extension request and it is still pending. Leaving on a later date than the date requested even while the extension application is pending could possibly lead to problems. I do not know the CBP attitude to that situation. If the I-539 is denied and I-94 already expired when your mother leaves, there is even more of a possibility that she will have further difficulties in returning. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.