You might also want to think about a rising share. Certain parts of your partner's costs will be fixed, and some will be variable. For electronic software distribution it's usually heavily skewed to the fixed costs. So a schedule where you get a rising percentage on increasing sales might work.

This only pays you more if your new version is as good as you think it is, so you're incented to write good code, and your partner gets his fixed costs covered quickly so he's incented to put effort into selling it.

Regards

Mark