[Sada]

static AmountMST openAmountMST(CustAccount _custAccount, CurrencyCode _currencyCode = '*')
{
    CustTrans           custTrans;
    CustPaymManTrans    custPaymManTrans;
    ;
    select sum(amountMST) from custTrans
        where custTrans.accountNum      ==   _custAccount   &&
              custTrans.currencyCode    like _currencyCode
        exists join custPaymManTrans
            where   custPaymManTrans.refRecId == custTrans.recId;

    return  -custTrans.amountMST;
}

SUM? просто заметил различия в коде