I entered into business transaction with a seller in US unfortunately the transaction was not successful and I asked for refund.

The man sent me postal money order since march and I just recieved it in Post office today, but to my disappointment the order happens to be for domestic cashing in US only. It is written on the order only for use in USA.

The seller has been doing funny things to me since I said I need refund.

What do I do now?