If you nevertheless picked up the chick and artificially nurtured, then you can not release it at will. Such a sparrow, nurtured by man, will not be adapted for free life, even if a flock of sparrows accept it. Practice shows that the sparrow released to freedom dies in 3-5 days.
So, if you already have a sparrow, prepare to stand for several weeks with the first rays of the sun, take it with you to work, in the evening stay up late. All in order to feed the bird every hour. Birds in general, and chicks in particular, should often eat.
Remember, feed the chicks you need strictly every hour, preferably every half hour, until saturation. If the feeding process is interrupted for more than two hours, the chick is likely to die. This is due to a specific digestive system and a very fast metabolism.
At night, you can safely sleep, because at night the metabolism of birds naturally and significantly slows down.
To choose the correct diet, you should try to determine the approximate age of the chick.