How do i negotiate a punishment with my mom?

my my took away snowboarding for this saturday. snowboarding is what gets me toned for volleyball and keeps me active.i love it to death! if i don't go, then im probably going to slack off. even though i go every saturday, snowboarding is important to me. it let's me clear my mind of what's going on around me. i got my ipod stolen at school like 4 months ago. i forgot to lock it up in my locker. (i know im stupid.) my mom told me not to bring it anymore. but yesterday i brought it and my friend wanted to see it and she forgot to give it back to me. she had it at her house. i told her on facebook just to give it back to me at school. my mom saw the comment and she told me no snowboarding because i brought it to school. i told her i was really sorry which i am but she doesn't want to change her mind. i'd rather her take my ipod away for 2 weeks than not going snowboarding. what is a good negotiation?