You could alternatively write two update statements..

UPDATE #tbl SET

 #tbl.Amount = #tbl.Amount + (SELECT Sum(T.Amount) FROM #tbl T WHERE T.RowNo < #tbl.RowNo)

WHERE #tbl.Type = 'S'

UPDATE #tbl SET

 #tbl.Amount = #tbl.Amount + (SELECT Sum(T.Amount) FROM #tbl T WHERE T.Type = 'D')

WHERE #tbl.Type = 'F'

HTH...