What you write and what query Agile Data builds you.

agile-data.php

<?php
// uses https://github.com/atk4/data

$c = new smbo\Model_Payment_Transfer($db);
$c->getRef('transfer_document_id')
    ->addFields(['other_side'=>'total_gross', 'other_account_currency'=>'account_currency','other_account'=>'account']);
$c->addCondition($c->expr('abs([total_gross])!=abs([other_side])'));
$c->addCondition($c->expr('[account_currency]=[other_account_currency]'));
$c->onlyFields([
    'system_id','system','id','doc_date','ref_no','account','other_account','total_gross','other_side',
    'account_currency','other_account_currency'
]);
echo $c->action('select')->getDebugQuery();

result.sql

SELECT `dochead`.`system_id`, 
       (SELECT (SELECT `s`.`legal_name` 
                FROM   `contractor` `s` 
                       LEFT JOIN `company` AS `s_c` 
                              ON `s_c`.`contractor_id` = `s`.`id` 
                WHERE  `s`.`deleted` = 'N' 
                       AND `s`.`id` = `s`.`contractor_id`) 
        FROM   `system` `s` 
        WHERE  `s`.`id` = `dochead`.`system_id`)            `system`, 
       `dochead`.`id`, 
       `dochead`.`doc_date`, 
       `dochead`.`ref_no`, 
       (SELECT `name` 
        FROM   `account` `a` 
        WHERE  `deleted` = 'N' 
               AND `id` = `_p`.`account_id`)                `account`, 
       (SELECT (SELECT `name` 
                FROM   `account` `a` 
                WHERE  `deleted` = 'N' 
                       AND `id` = `td_p`.`account_id`) 
        FROM   `dochead` `td` 
               LEFT JOIN `payment` AS `td_p` 
                      ON `td_p`.`dochead_id` = `td`.`id` 
        WHERE  `td`.`deleted` = 'N' 
               AND `td`.`doc_type` = 'payment' 
               AND `td_p`.`transfer_document_id` IS NOT NULL 
               AND `td`.`id` = `_p`.`transfer_document_id`) `other_account`, 
       `dochead`.`total_gross`, 
       (SELECT `td`.`total_gross` 
        FROM   `dochead` `td` 
               LEFT JOIN `payment` AS `td_p` 
                      ON `td_p`.`dochead_id` = `td`.`id` 
        WHERE  `td`.`deleted` = 'N' 
               AND `td`.`doc_type` = 'payment' 
               AND `td_p`.`transfer_document_id` IS NOT NULL 
               AND `td`.`id` = `_p`.`transfer_document_id`) `other_side`, 
       (SELECT (SELECT `name` 
                FROM   `currency` `c` 
                WHERE  `id` = `a`.`currency_id`) 
        FROM   `account` `a` 
        WHERE  `a`.`deleted` = 'N' 
               AND `a`.`id` = `_p`.`account_id`)            `account_currency`, 
       (SELECT (SELECT (SELECT `name` 
                        FROM   `currency` `c` 
                        WHERE  `id` = `a`.`currency_id`) 
                FROM   `account` `a` 
                WHERE  `a`.`deleted` = 'N' 
                       AND `a`.`id` = `td_p`.`account_id`) 
        FROM   `dochead` `td` 
               LEFT JOIN `payment` AS `td_p` 
                      ON `td_p`.`dochead_id` = `td`.`id` 
        WHERE  `td`.`deleted` = 'N' 
               AND `td`.`doc_type` = 'payment' 
               AND `td_p`.`transfer_document_id` IS NOT NULL 
               AND `td`.`id` = `_p`.`transfer_document_id`) 
       `other_account_currency`, 
       `dochead`.`contractor_from`, 
       `dochead`.`contractor_to`, 
       `dochead`.`vat_rate_id`, 
       `dochead`.`currency_id`, 
       `dochead`.`vat_period_id`, 
       `dochead`.`journal_spec_id`, 
       `dochead`.`job_id`, 
       `dochead`.`nominal_id`, 
       `dochead`.`doc_type`, 
       `dochead`.`deleted`, 
       `dochead`.`created_by`, 
       `dochead`.`deleted_by`, 
       `dochead`.`updated_by`, 
       `_p`.`payment_type_id`, 
       `_p`.`account_id`, 
       `_p`.`account_reconcile_id`, 
       `_p`.`bank_statement_id`, 
       `_p`.`transfer_document_id`, 
       `dochead`.`id`                                       `_p` 
FROM   `dochead` 
       LEFT JOIN `payment` AS `_p` 
              ON `_p`.`dochead_id` = `dochead`.`id` 
WHERE  `dochead`.`deleted` = 'N' 
       AND `dochead`.`doc_type` = 'payment' 
       AND `_p`.`transfer_document_id` IS NOT NULL 
       AND Abs(`dochead`.`total_gross`) != Abs((SELECT `td`.`total_gross` 
                                                FROM   `dochead` `td` 
                                           LEFT JOIN `payment` AS 
                                                     `td_p` 
                                                  ON `td_p`.`dochead_id` = 
                                                     `td`.`id` 
                                                WHERE 
                                           `td`.`deleted` = 'N' 
                                           AND `td`.`doc_type` = 'payment' 
                                           AND `td_p`.`transfer_document_id` IS 
                                               NOT NULL 
                                           AND `td`.`id` = 
                                               `_p`.`transfer_document_id`)) 
       AND (SELECT (SELECT `name` 
                    FROM   `currency` `c` 
                    WHERE  `id` = `a`.`currency_id`) 
            FROM   `account` `a` 
            WHERE  `a`.`deleted` = 'N' 
                   AND `a`.`id` = `_p`.`account_id`) = (SELECT (SELECT 
           (SELECT `name` 
            FROM 
                   `currency` `c` 
                            WHERE 
                   `id` = `a`.`currency_id`) 
                    FROM   `account` `a` 
                    WHERE  `a`.`deleted` = 
                           'N' 
                           AND 
               `a`.`id` = `td_p`.`account_id`) 
                                                        FROM   `dochead` `td` 
               LEFT JOIN `payment` AS 
                         `td_p` 
                      ON `td_p`.`dochead_id` = `td`.`id` 
                                                        WHERE 
               `td`.`deleted` = 'N' 
               AND `td`.`doc_type` = 
                   'payment' 
               AND `td_p`.`transfer_document_id` IS NOT NULL 
               AND `td`.`id` = `_p`.`transfer_document_id`)