Thinking of it, if they arent too big a band then I agree with Lasse...
It's not suprising to me that others would want to buy your music - your music is seriously good shit Ola.
You should charge by the note.
Cool. Negotiate whatever you find fitting as an up-front payment and then insist to be given compositional credits for the song, so you'll get the deserved royalties.
Must be a pretty cool band (as in not douches) too..I'm just suprised they actually PMed you to ask you to use one of your riffs. Most bands would just fucking take it, it must be a pretty cool riff.
Ola: to be honest, there is absolutely no legal, ethical or moral reason why you can't name a band that is interested in your riff!
Unless you think this would somehow lead them to not want to buy your riff ...
I'm not telling what band it is until the deal is closed.