My Cat Caught a mouse

My cat caught a mouse and it has a broken leg i have had it for about 3
hours now in my old fish tank. I have never cared for mice before and
my cats are good hunters so if i let it go i dont know if it will
servive. I dunno if it is a girl or boy or anything but he/she is
draging his/her leg around and i am hopeing it will heal then i will
let him or her go. Any suggestions on ways to help the leg heal??? or
how long it will take??? or if i do let him/her go will it be able to
survive in the wild again or should i let them go now??

  Hattie