Based on its omission from the OSR's EntityFilter description, I don't think it's possible to filter an Invoice Query Request by Sales Rep Ref. You can filter by Transaction Date Range using TxnDateRangeFilter and FromTxnDate by itself or in combination with ToTxnDate.
It's possible to filter a Response by the SalesRepRef (with ListID or FullName) using XPATH, but there's enough complexity to doing so that I'd avoid it in the absence of a truly compelling reason (super-high transaction volume, for example.)
Creative Solutions Incorporated
14000 Creekside Drive
Matthews, NC 28105