you really don't need to store that data in your master table, that would be very bad design.
Just calculate it in your query that fetches the data from the master like this
( select sum(isnull(SaleItem.PriceItem, 0))
where SaleItem.SalesID = Sales.SalesID
) as TotalPriceItem
Now you will always have the correct value everytime you fetch this query and never have to worry about keeping the value in sync, or about transaction, or whatever
If you really must store the value in Sales (which is bad design) then do NOT EVER do this in a client because you cannot keep the value in sync. In that case make a trigger on the SalesItem table that will calculate the value and update the Sales table.