504 Error

If you decide to map the table occurrences to your FileMaker tables rather than using LedgerLink's, pay attention to the validation assigned to the qbo_id. A transaction's parent and line field definitions for the qbo_id are different. 

Unlike the parent record for a transaction, the line item's qbo_id should NOT be set to unique validation.

Line qbo_id definition:

Parent table qbo_id SHOULD be validated as unique.

If the setting is incorrect, you may receive this error response during sync;

{ ...//other object properties

"descriptor" : "Committing the record", "errorCode" : 504, "errorMessage" : "Value in field is not unique as required in validation entry options", "scriptName" : "_Commit Record ({Entity, Record} )", "type" : "FM"}