I'm very sorry if your dog is not doing well.
The only approved, humane and safe way to lt our loved ones go when they are suffering is through euthanasia, which is administered by a licensed veterinary individual. The process works by letting the dog go into an unconscious sleep state (anesthesia) and then continuing to administer this anesthetic through the vein until breathing and heartbeat ceases. It is a calming way to go. Your vet can certainly discuss how it works with you again beforehand, and if you need help with payments, you can look into local shelters and humane societies, as well as this no to low interest credit card for vetcare: www.carecredit.com
Best of luck, and again, I'm so sorry for your loss.
Please reply if I can help you further,