IICS Cloud Integration: Is there any way to use a child object in a REST API call through Service Connector?

Question

When using a child object (relatedInvoices in the below example) in the REST API call in a Service connector, I get an error "Unable to parse the payload". The same API call in postman is successful, but not in IICS. Pls let me know if there is any fix for this or is there any other way we can approach this -

Method: POST Request body ex:

{{
"PaymentNumber":"{$PaymentNumber}",
"PaymentDocument":"{$PaymentDocument}",
..
"relatedInvoices":
{
"invoiceNumber":"{$invoiceNumber}",
'invoiceAmount":"{$invoiceAmount}",
..
}
}}