Unfortunately none of the causes of this kind of fluid in the abdomen (ascites) is good. Dogs can get ascites secondary to heart disease, or tumors in the abdomen, or kidney disease i.e: protein losing enteropathy (not enough protein being held by the circulatory system to hold fluid in the vessels). In order to get an accurate diagnosis X-Rays would have to be performed to see where the problem is occuring. Often if there is fluid in the abdomen there may be fluid in the chest, so your vet would really need X-Rays of the chest and abdomen in 2 views of each cavity for a total of 4 X-Rays to determine if, for example, the heart in enlarged or abnormally shaped, or if there are any organs in the abdomen that are enlarged or of unusual shape. Any organ can be envolved. The fact that the analysis of the fluid did not show cancer cells is an optimistic finding, since cancer is high on the list, however false negatives are not unknown. If it is due to heart disease there are many medications that can help control the problem with a good quality of life. Therefore you should probably have the tests performed that are deemed appropriate by your vet. Explain that money is an option and ask if she believes that it could be something controllable. If your vet doesn't believe there is any hope for control you can decide from there. Additionally, ask her if she can use Care Credit. Care Credit is a interest free credit card for medical or veterinary purposes only. Since it is interest free you will be able to pay off the vet bills immediately with Care Credit, but will be able to pay off the Care Credit credit card each month with out penalty. Care Credit is also available online. Please let us know the outcome!
If it is heart disease please know that my Tiffy went 5 years with advanced heart disease. She was diagnosed at about 8 and lived to 12. I recently had to put her down due to kidney problems that were aggrevated by an attack by another dog.(boy, do I miss her!) Her heart was very enlarged and her liver was also for a long time. But with the right medication her lungs remained pretty clear and she lived a comfortable happy life. She never had fluid in the abdomen, so I don't know about that. But the heart meds they have these days does wonders for dogs with heart disease. I wish you the best.