If the integration to Salesforce is throwing errors with this message, "INVALID_FIELD_FOR_INSERT_UPDATE", it's likely because the object.property being passed thru the API doesn't exist -- or if it does -- isn't visible via permissions.
Frequently, we receive this in the NPSP accounts for the field, npsp__Primary_Contact__c.
The field exists in the "setup" -> "build" -> "customize" -> "opportunities" -> "Fields". But if you click on the field name, "Primary Contact" and then on "Set Field-Level security", you'll see that the permissions may be all un-checked.
Check appropriate roles (Sys Admin & Standard User) -- and now this field is visible thru the API. And you will no longer receive this error message.