You can create a global varialbe for Sales Header and find the record using GET function like
SalesHeader.Get("Sales Line"."Document Type","Sales Line"."Document No.");
You need a Date field in Sales Line table to insert Document Date fied of Sales Header as your temp table is based on Sales Line table.
ex: Shipment Date
then code sample will be
TempSalesLine."Shipment Date" := SalesHeader."Document Date";