All you can declare is what you know for a fact is true. In this case, you can declare that you have not received the money order (and, obviously, that you have not cashed it.) Do not declare anything about what your tenant did or did not do - you have no way of knowing that.
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I agree with my colleague; do not sign any declaration that is not true or for matters that are not withing your personal knowledge. Why is the tenant seeking such a declaration from you? Are you in the process of evicting the tenant? The tenant should report the money order as lost and try to obtain a refund.