Cat ate old food, now panting & drooling. Is it an emergency?

Oh no, poor Ezra! It sounds like this food has made him sick, and it may be causing a toxic reaction. It's not normal at all for a cat to pant, and he really does need veterinary attention now. There's nothing you can do at home for him. I would see if you can borrow money from family and friends to afford an ER visit, and I can give you some resources that may help you afford a vet. You can see if your vet accepts payment plans, if there is a low cost clinic in your area, or you can apply for Care Credit online. There is also an organization called Red Rover that give grants for treatment of sick animals, and you can see if you qualify for one of their grants. Best Friends also has a list of resources that offer financial assistance on their website.
http://www.humanesociety.org/animals/resources/tips/trouble_affording_pet.html?credit=web_id91754962
https://www.redrover.org
https://www.carecredit.com
http://bestfriends.org/resources/financial-aid-pets
http://lessonsfromaparalyzeddog.com/when-pet-owners-cant-afford-veterinary-bills/


The Feline Veterinary Emergency Assistance Program is a nonprofit 501 (c)(3) organization that provides financial assistance to cat and kitten guardians who are unable to afford veterinary services to save their companions when life-threatening illness or injury strikes.

God's Creatures Ministry helps pay for veterinarian bills for those who need help.

The Onyx & Breezy Foundation has many programs including helping people with medical bills. They are a good resource for information.

Brown Dog Foundation provides funding to families with a sick pet that would likely respond to treatment, but due to circumstances, there is not enough money immediately available to pay.
IMOM is dedicated to insure that no companion animal has to be euthanized simply because their caretaker is financially challenged.

I hope this was helpful and one of my resources helps you afford the vet. Please take him into an ER vet clinic; if there isn't one in your area, there will be a vet on call for emergencies. The information to reach that vet is found on a clinic's answering machine or website.