Jonas bails himself out of a grounding

This morning Jonas got grounded for giving “push-back” to Sandy and me and lost a trip to see A Christmas Carol in IMAX. After the anger, tears, and tantrums, he offered us a heartfelt apology, then mopes off to his room to serve out his sentence while Sandy and Amelia got ready to go. So now Sandy and I are feeling like maybe we should let him go since he seems truly sorry for his actions, but don’t want him to think an apology will fix extremely poor behavior and there aren’t consequences. As parents how do we remain tuff, fair, and compassionate in this situation? Sandy and I came up with what we think is a reasonable compromise; he would have to use his allowance money to buy a ticket. What do you think? Were we to easy or not tuff enough? How would you handle it?