If it was a postal money order, the postmaster can dig up a record of the sale. If it is another type, maybe the manager of the place it was bought could dig up a record of the sale. Short of that, there is no resolution unless the post office somehow lost 2 items, or somehow on the other hand the money orders were forgotten or mis-placed. I know If I was the accused, I would go out of my way to try and prove payment.