Why don't you change it to:
Post as you wish, each post will be paid $X and the maximum per week will be Y.
Any post with no significant quality / value won't be counted.
Hhampuz already beat me to it but I'm gonna top it up- I've said something like this somewhere I can't remember.
Giving the freedom to post any number of posts per week is good, but hiring someone who will only write 5-10 quality posts per week will only benefit the forum and not the client side - the client is looking for numbers (to have their signature display many times per week), so as a manager, you must strike a balance between satisfying your clients and ensuring the forum is not polluted. It's a difficult job.