I guess you can just ask your employer to give you an NOC for your sponsoring of your mother.

In that letter, aside from their no objection of bringing your mother here, will also indicate your current salary. No need to go to labor office, since your company issuing NOC is as clear as the daylight that you are working for them.

I have done that, since I have brought my sister here, and when I applied for family visit visa then, they asked my company NOC for my family member application.

By the way, family member applications ofr visit visa must be done at IMMIGRATION OFFICE at Gharaffa.

If your wife is here, bring the passport copy and Qatar ID of your wife, to prove that your family, aside from your mother is already here.

Good luck