Postponing surgery?

It depends. I assume the vet has examined it. Did he/she say that it felt like a fat deposit rather than a cancerous mass? Or did they say it was unclear?

If it's not cancerous and not in a dangerous area/not causing pain then there actually is no reason to remove it at all. All three of my dogs growing up had benign fatty masses in them. They've had them for years and are still alive and well. The masses have not grown at all. People get benign tumors or cysts as well. The only time you want to remove something that's obviously benign is if it's causing pain (which they can sometimes do if they're putting pressure on nerves), or if it's in a bad spot like the brain where any foreign body/pressure could cause problems. Otherwise there's no point in performing invasive surgery to remove something harmless. Especially in an elderly cat. Anesthesia would be significantly more dangerous to an elderly cat than a benign fatty tumor.

If your vet feels confident that it's benign, then I wouldn't worry, and his/her advice to "wait and see" is totally reasonable. If your vet -can't- tell by feeling it, then I understand your concerns, but I imagine as long as it's not changing size, a two week delay wouldn't be unreasonable.

Is there any way they can biopsy the tumor to find out if it's cancerous or not, before taking the more invasive step of removal, if there's uncertainty? I don't really know how that procedure goes for pets and if they still have to be anesthetized for a biopsy or not (people don't).