How to pull a custom field from accounts in the transaction?

Use the payroll method GetList().

$arOrder = ["ID" => "DESC"]; // sorting
$arFilter = ["UF_DEAL_ID" => $dealId]; // filter by ID of the transaction
$arSelect = ["ID", "UF_DEAL_ID"]; // what fields to choose

$invoices = CCrmInvoice::GetList($arOrder, $arFilter, false, false, $arSelect);

while($invoice = $invoices->Fetch()) {
 // your code
}

This problem can occur if the transaction multiple accounts (actually, so cycle is used), so you will need additional filtering options.

You can also use the rest method crm.invoice.list. But you will need to additionally generate incoming webhook and use HttpClient to invoke a method

in this case, the code will be like this:

use Bitrix\Main\Web\HttpClient;
use Bitrix\Main\Web\Json

$client = new HttpClient;

$webhook = "https://webhook_url/crm.invoice.list";
$params = [
 "filter" => $arFilter,
 "order" => $arOrder
];

$request = $client->post($webhook, Json::encode($params));
$result = Json::decode($request);

foreach($result["result"] as $res) {
 // your code
}