My cat has liver problems and a m in his abdomen?

I have had my cat for a long time now , and when most people tell me he is just getting old thats not good enough for me. This cat is my buddy and he doesnt seem to be miserable or in pain cause he still purrs and looks at people with big open eyes. But I did take him to the vet yesterday and let them do blood work and xrays. So the bloodwork says that everything is normal , blood , oxygen , electrolytes , kidney.. all that is fine but the liver isnt.and there is a m, most likely a tumor in his abdomen and they said that unfortunately there is nothing they can do because they cant tell what exactly is wrong with the liver until they do an ultrasound. problem is i got laid off my job a few months ago , and just paid $300 for the visit yesterday. an ultrasound is big bucks!! but i want my cat to be healthy again. what do i do??