It's a BOY, congratulations!

Oh... Sorry But yep, he's definitely a boy. His urethra is much bigger and the distance between that and his anus is bigger than a female's is.

Take the mouse back and smack the store owner with it(don't hurt the mouse though, LoL). It's sad when pet store workers can't even tell the difference between males and females(especially in rats!). I mean jeez! 'Dem things are quite obvious, LoL!