The problem i think is your contract.
You must make sure that you are clear about payment and your delivery.
The best thing is that you stated in your contract that you need full payment within XX days. or alse you are selling your product to others and that if he did pay half that you will send it back minus the cost that you had to compleet his order. that include storage collecting and other costs.
If that is not in your contract the only thing you can do now is sending by courier a demand letter that he MUST pay you within 7 banking days or you are forced to send his money back but minus the cost that you had.
good luck,
Bertus
Horesca