The subcontractor would have to write the SDS and give it to Sigma.  What happens next depends.

If Sigma resold the material without repackaging it, they would be acting as a distributor and they can simply pass along the SDS they received from the subcontractor. This is the concept of "downstream flow" under the HazCom Standard:  http://www.ilpi.com/msds/faq/partb.html#downstream  However, by doing this they reveal the source of the chemical, something they may not wish to do.

If Sigma put their own label on the bottle then, because the label and SDS have to correspond to each other and they would be replacing the subcontractor's name on the sheet, Sigma would become the "responsible party" on that SDS and would be responsible for performing their own hazard classification and issuing a new SDS with their name and product info. They shouldn't/can't simply rely on the one the subcontractor gave them as Sigma is now legally responsible for the accuracy and completeness of the SDS. Ideally, Sigma would have to redo the classification themselves, although the task is certainly made easier if the subcontractor did a good job. See http://www.ilpi.com/msds/faq/partc.html#contact for example.

 Of course, that task would be extra easy if SDS's were required to include a complete bibliography that lists original, *primary* sources of each datum. That's never going to happen, but I always recommend that SDS authors document and retain the research they used in constructing the sheets so they can show they did their due diligence with the data available to them at the time. BTW, there is nothing in GHS or HazCom that prevents SDS authors from including footnotes or a bibliography!

Rob Toreki