[sukhanchik]

А так?

    select sum(AmountMST) from custTrans
    where custTrans.AccountNum == "165057137"
            && custTrans.TransDate <= 01\01\2010
            && custTrans.PostingProfile == "62.10.04"
    join RContractTable
        where RContractTable.RContractCode == custTrans.RContractCode &&
              RContractTable.RContractAccount == custTrans.RContractAccount &&
              RContractTable.RcontractPartnerType == RcontractPartnerType::Cust
              RContractTable.RContractAccount == '12КР002008'
    exists join ledgerTrans
          where ledgerTrans.AccountNum == "62.10.04" &&
                   ledgerTrans.Voucher == custTrans.Voucher &&
                   ledgerTrans.TransDate == custTrans.TransDate;

Или так:

    select sum(AmountMST) from custTrans
    where custTrans.AccountNum == "165057137" &&
              custTrans.TransDate <= 01\01\2010   &&
             custTrans.PostingProfile == "62.10.04" &&
             custTrans.RContractCode == 'Группа' &&
             custTrans.RContractAccount == '12КР002008'
    exists join ledgerTrans
          where ledgerTrans.AccountNum == "62.10.04" &&
                   ledgerTrans.Voucher == custTrans.Voucher &&
                   ledgerTrans.TransDate == custTrans.TransDate;