SQL 2005 Stored Procedure

I have a SP that Picks Sales O)rder lines and updates the stock with Qty Picked.

If the user picks more than the available stock it then goes negative. Is there an eay way to just set StockQty to zero?

If @PickType = 'PickMan' 
	Begin
		Update OrderDetail Set QtyPicked = @PickQty, Pickedby = @pUser, DatePicked = GETDATE ( )
		Where OrderNo = @OrderNo and Barcode = @Bcode
		set @iMsg = 'Picked ordered qty for ' + @Bcode
 
		-- update the stock qty
		Update Stock Set StockQty = StockQty - isnull((@PickQty),0), 
		QtyReserved = QtyReserved - isnull((@PickQty),0)
		Where Barcode = @Bcode
		goto ShowResults
	End

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