Tummy tuck, 9 months post op. I am having yellow discharge with odor. What could this be? (Photo)

As the other doctors have said this could certainly be a stitch which has worked its way to the surface and be a stitch abscess. However I notice that you have raised hypertrophic (even possibly keloid) scar formation. I would recommend a course of steroid injections to have flatten these scars. However if you have already been having steroid injections then the white discharge could be a deposit of the steroid working its way through to the skin.

I'm sorry you are having this issue. Yes, I would agree that you likely have a suture/stitch that has bacteria attached to it and your body is trying to fight that (granuloma). It needs to be removed which may require a surgical procedure. A culture swab of that fluid that is negative for bacteria is not enough proof that that there is not infection. If you've taken antibiotics recently or the swab was not done correctly, the results may not be accurate. You need a plastic surgeon to see you. Best wishes.

It is likely a stitch that is causing your issues.  If you remove it, your issues should resolve.  The procedure is relatively simple with minimal recovery.