Can I use my I-797, after filing my I-90, as proof of my employer to maintain employment.

Speak to your employer.

Unfortunately, we are not allowed to provide any advice as to what you or your employer should do in this case. You can speak to your employer directly to see if they will accept proof of filing as valid proof of your green card status, or If you require an extension on your green card, you can make an appointment to speak to the USCIS at a local office where you live. CITATION: Page 98; http://www.uscis.gov/sites/default/files/USCIS/About%20Us/Electronic%20Reading%20Room/Customer%20Service%20Reference%20Guide/Permanent-Residents4-3.pdf