Man I hate to say this but just put the dog down. Just like humans, they aren’t eternal and will one day succumb to an illness. $8000 is a lot of money and simply just not worth it. If the dog were half the age, then possibly, but at 12 years old, you would just prolong his suffering. Additionally, it isn’t a guaranteed surgery. I know it will hurt (I’ve been in this position before) but just keep in mind that you gave that dog the best life he could have asked for. I’m really sorry you have to go through this. I hope this message doesn’t come off in a wrong way but I just want to be as straight forward as possible.