Problem with SET variable statement in Stored Proc

ALTER PROCEDURE [dbo].[proc_Update_OP_Single_Week] 
	-- Add the parameters for the stored procedure here
	(@RetailWeek varchar(6),
	@MinDate datetime,
	@MaxDate datetime)
AS
SET @MinDate = (SELECT MIN(Date) FROM tRetailWeeks WHERE AgrWeek = @RetailWeek)
SET @MaxDate = (SELECT MAX(Date) FROM tRetailWeeks WHERE AgrWeek = @RetailWeek)
BEGIN
	-- SET NOCOUNT ON added to prevent extra result sets from
	-- interfering with SELECT statements.
	SET NOCOUNT ON;
    -- Insert statements for procedure here
SELECT  Right(sales_person,5) AS SalesPer5Char,branch_code,reg_no,tran_no,tran_date,SUM(sales_qty) AS Quantity,SUM(sales_value) AS Net_Amount, SUM(tax_value) AS Tax_Amount
FROM 
SQLIIS.rstar.dbo.saleanalysis
WHERE sales_value<>0 AND tran_date>@MinDate AND tran_Date<@MaxDate
GROUP BY Right(Sales_person,5), branch_code, reg_no, tran_no, tran_date
ORDER BY tran_date
END