Hello, Dorothy! Thank you for helping out the turtle. Red-eared sliders are easy to identify because they have a red mark on their cheeks. If the turtle doesn't have that, it's not a red-eared slider. If you send photos of the turtle, top, bottom, and side of the head, I can give you my opinion on what it might be. Sometimes I know for sure but not always as there are a lot of similar US turtles (I assume you're in the US; I'm not very familiar with species in other countries).
Since the poor turtle is missing an eye and part of the jaw, he/she is not a good candidate for release. Not only will predators get him/her more easily but he/she will have more trouble finding foods. Plus, he/she has gotten used to some commercial food by now. How big is the turtle? If he/she is over 5" long, I may be able to sex the turtle for you if you can send a photo of the front nails. Males have much longer nails.
My turtle pages are indexed at
http://www.fishpondinfo.com/turtles/index.htm and would cover most of what I would say if my time was limitless. If you really want to release or find a good home for the turtle, then I suggest contacting your local reptile or turtle organization. If you tell me what state you're in, I may be able to find one for you. They are better able to ascertain if the turtle can be rehablitated or released.
Good luck!
P.S. Make the turtle's day and get him/her some live mealworms! When I buy mine (for my turtle and chickens), I often toss the store red-eared sliders a few. They go nuts.