First off euthanasia should be done by a qualified veterinarian. If you cannot afford to have your rat euthanized several vets can have your sign him over to their hospital and provide the euthanation for free. Because of his age and the location of the tumour surgery is most likely not an option. There is a product on the market called Pau d'Arco that helps slow tumour growth. You can ask about it on the Rats Rule forum. The link is:
http://www.goosemoose.com/rfc/index.php
Before you decide to euthanize him you may want to make sure it's really a tumour and not a sebacious cyst or an abscess, which can be easily cured. You can also call around and see if any vets will take payments or offer to volunteer to work off your bill. You can contact your
local Humane Society as well and see if there are any organizations around your area that help pay for vet bills. The sooner you get him to the vet the sooner treatment can begin.