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...