Just make a demand and stick to it, or lower it slightly, if he won't pay, kindly take him down to 10% hull and see if he'll reconsider.
If I see a trader in a 5K full of ore, and he uses the old "I have no money, I just bought this ship"... if he's lying, he dies, if he's telling the truth, he dies, if they are stupid enough to haul ore with no money to pay pirates, then they deserve to be punished.
I know you're the probably the trader here, so I don't know why I'm talking to the pirate.
