還有你交房租的時候不也應該得到1張收據?
USPS的money order 不可以stop-payment.不過你可以去file a form that will replace the money order if it has not been cashed within 60 days. And you can also file an affidavit if the money order has been cashed by someone other than the original payee. However, the thing is that the lack of the evidence or any other info regarding that money order would not help you even if you can get a lawyer.
如果是我的話,我會再去找相關的receipt再去郵局問問.