Query Optimization required - Subquery using Inner Join
You could rewrite with a LEFT JOIN:

SELECT
  IT.[iInternalTransactionID]
, IT.[iInternalAccountID]
, C.[iContractID]
, IT.[dtTransactionDate] AS TransactionDate
, (CL.[sFirstName] + ' ' + CL.[sLastName]) AS ClientName
...
-- ### unchanged up to here ##--
, G.TermNo
, G.TermNo * G.MaxMonthlyAmount AS TotalAmount
-- ### end ### --

FROM 
    [tbl_InternalTransactions] IT
    INNER JOIN [tbl_Contract] C ON C.[iContractID] = IT.[iContractID]
    INNER JOIN [tbl_Client] CL ON CL.[iClientID] = C.[iClientID]
    INNER JOIN [tbl_FinanceStructure] FS ON FS.[iFinanceStructureID] = C.[iFinanceStructureID]
    INNER JOIN [tbl_InternalAccount] IA ON IT.[iInternalAccountID] = IA.[iInternalAccountID]
    AND IA.[iInternalAccountID] = @InternalAccountID

--  ### the two subqueries converted to a LEFT JOIN ###
    LEFT JOIN
      ( SELECT CPH.iContractID
             , CPH.dtTransactionDate
             , MAX(CPH.iTermNo) AS TermNo
             , MAX(CPH.cMonthlyAmount) AS MaxMonthlyAmount
        FROM tbl_ContractPaymentHistory CPH
        WHERE CPH.iInternalAccountID = @InternalAccountID
        GROUP BY CPH.iContractID
               , CPH.dtTransactionDate
      ) G
      ON  G.iContractID = C.iContractID
      AND G.dtTransactionDate = IT.dtTransactionDate
-- ### end of changes ###

WHERE 
    IT.[dtTransactionDate] &gt;= @FromDate
    AND IT.[dtTransactionDate] &lt;= @ToDate
...


or using OUTER APPLY:

--  ### the two subqueries converted to an OUTER APPLY ###
    OUTER APPLY
      ( SELECT MAX(CPH.iTermNo) AS TermNo
             , MAX(CPH.cMonthlyAmount) AS MaxMonthlyAmount
        FROM tbl_ContractPaymentHistory CPH
        WHERE CPH.iContractID = C.iContractID
          AND CPH.iInternalAccountID = @InternalAccountID
          AND CPH.dtTransactionDate = IT.dtTransactionDate
      ) G
-- ### end of changes ###




These are just rewritings along the lines you tried. Not at all sure if they improves efficiency. They will probably do but the main bottleneck may be elsewhere (indexes).