i797 expiring early than h1b

My i797 is expiring in sep 16 and i94 is expiring in jul 17. My h1b amendment plus extension is in progress.
Now i m planning to go to canada for tourism. I want to check while coming back to usa will immigration officer check my i797 and reduce my i94 to sep 16.

Yes, they can reduce it to Sep 16. They are usually kept in sync and I am not sure whey your I-94 was expiring 1 year after 797.

BTW, if you leave US while extension is progress then there could be an impact - extension of status would be abandoned. Amendment and 797 extension may still get approved but w/ consular processing and not I-94 extension.

Did you talk to your attorney about your travel plans?