If you call the emergency vet, they sometimes give advice over the phone, though it may be limited due to liability. It may vary depending on the situation and the vet on duty. You can also google what to do if the dog eats the poison and may find a DIY remedy, but your best bet by far is the vet. You'll need to know your dog's weight, what the poison was made out of -- there are two or three different types -- and when he may have eaten it. If the dog ate enough rat poison to kill him it should take 3-5 days for him to die.
Some emergency vets can set up a payment plan or give credit. The treatment for poisoning may not be as much as you think. You could also determine how far you are willing to go based on finances, among other things. Does your dog have a regular vet, someone you have known for a long time that might be able to provide a payment plan? Could you borrow money from a friend or relative?
If the dog does survive, you should consider finding him a new home through a reputable rescue, preferably a Lab rescue. A dog should not live outside or be tied up in a garage and it sounds like you're not in a position to adequately care for the dog at this time.
I hope things turn out OK.