I'm so sorry that what I've said and what they've said are two different things. I honestly do not know why they are refusing to do a needle aspirate on the lump and are just wanting to jump to surgery.
Before I answer, I'll ask if your vet was talking about full surgical removal of the lump, or were they talking about a biopsy (taking just a small piece of the mass)? Did they mention if they would put your dog under general anesthesia or just sedation?
If they were talking about full surgical removal of the lump under anesthesia, here is my answer:
During my time as a student in vet school, I spent a three week block of time doing nothing but oncology -- in other words, we saw lots of dogs and cats every day that had lumps and tumors everywhere, outside and inside. After performing a basic physical exam on them, our very next step was to perform a needle aspirate on each and every lump we could find on their body. One dog I saw had nearly 25 lumps and I had to aspirate each and every lump, put each sample onto a slide, stain the slide, examine it under a microscope, and list the findings in the record. This was only one year ago so it is not old-fashioned. Our clinicians (board certified veterinary oncologists) told us that there were no lumps that should not be aspirated, and also told us that you never remove a lump without knowing what it is first. If this even is a tumor (which we don't know for sure yet), there are different types of tumors which must be addressed in a very specific manner to be correctly removed surgically. In other words, a mast cell tumor and a fibrosarcoma should be approached in different ways -- certain tumors must be removed with a 2 cm margin all the way around, others are different. Again, there were absolutely no tumors that our clinicians instructed us to 'just remove'.
I completely understand how difficult of a situation this is for you. Your vet who you know and can talk to in person is telling you one thing, and I am telling you another over the internet. And you are just concerned about your wonderful dog and want to do the right thing. My suggestion would be to call another vet in your area (or two or three) and speak with them about your situation. If they won't talk to you, they are not worth trusting your dog's care with. You have a right to ask them about whether or not they would perform a needle aspirate or biopsy before surgical removal. You also have a right to ask your own vet to specifically tell you why they won't do this.
Please let me know if you find out exactly what your vet is planning -- biopsy vs surgery, anesthesia vs sedation. My answer above is based on if they are wanting to fully remove the lump without knowing what it is first.
I'm so sorry, again, that you have to live with such a fear of losing your beautiful dog. Please consider calling other vets in your area to at least speak to them about what they would do for your dog, and if they could do it sooner, too. I'm here for you, too. I'll help you as much as I possibly can! I completely understand and can sympathize with the fear you're feeling right now. Don't give up up hope, and don't settle for anything or anyone that doesn't make you feel confident and comfortable about what's being done for your dog, Gizz. Please keep me updated. I'll be standing by to hear from you!