well, now you've got a real problem. you don't know what you've got nor do you know how to care for it. since it wasn't taken from the wild, releasing it 'back' to the wild is definitely not a good idea. the problem is that while you may successfully maintain the tadpole to the point where it becomes a 'frog', the likelyhood of it living very long as an adult frog in a ten gallon tank is poor.
neither frogs such as this, nor the vast majority of turtles, either of which may be sold in pet shops, survive very long in captivity. frogs and turtles almost always live an unhealthy and short life in an aquarium due to inadequate care. in my opinion and in the opinion of most zoologists, neither turtles or frogs are appropriate pets for those without a full understanding of the tremendous amount of care they require.
The care and feeding is more complicated than is generally thought ---> daily maintenance of the enclosure, enclosure apparatus and feeding are alot of work. most people are dismayed to find that they can't just stick a turtle or frog in a box or tank of water or let them loose in their yard, tossing lettuce to it once in a while.
It is illegal in the U.S. for pet stores to sell turtles less than four inches in length (this is problematic for those species whose full adult size is 4" or less!) and it may be illegal for them to sell frogs or tadpoles as well. it depends on which species and in what state.
As with all wild-caught amphibians or reptiles, the animals found in pet stores have been under stress for some time. As a result, they are most likely suffering from protozoan and bacterial infections, including Salmonella which is easily transmitted to young children. Additionally, they are usually emaciated and dehydrated due to long periods of time without food or water or being held in areas too cold to stimulate the appetite.