I am currently employed by Company A, Company B has offered me employment, but only when I get a green card(they don't do H1s). Company B has a pre-approved labor which they are willing to use and file a green card for me, so that I can join them after I get the green card. Is it ok for company B to file I-140 with out me being employed by them at present?

Yes, company B can file the I-140 even if you are not currently employed there.