Maybe something along the lines of
SELECT WEEK(Rtv_Head.RtvDate) as nWeek, MIN(Rtv_Head.RtvDate - Dow(Rtv_Head.RtvDate) + 1)) as dWeek_Start, ;
SUM(Qty*Price) AS Ret_Total ... from ... group by 1
Completely untested and without giving it a second thought, so it might be completely wrong
[Update]I wrote Week instead of Dow, just corrected it
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