I've never done Party Poker, but I tried to cash out of Paradise under similar circumstances.

I had $32.16 (I'm making numbers up to make my point)
I tried to withdraw $30, and it said that I couldn't. I had to either withdraw everything, or a $50 minimum. SO I withdrew $32.16 and it went through fine.

Is it possible that you are trying to withdraw slightly less than your total amount?