unprocessedProjectCosts - reverse Supplier Invoice cost variance items

Summary:

Unable to reverse Supplier Invoice cost variance items in Project Cost - either by reversal/adjustment api or by unprocessedProjectCosts

Content (required):

We need to build something that can reverse PA cost variance items that have been created and accounted from Payables. The reversal rest API will not allow this: You can't perform this adjustment because the document entry of the expenditure item doesn't allow reversals. (PJC-2220492)

Instead I am trying to create a negative item using rest api unprocessedProjectCosts. Problem is to get the finance accounting right. If passing the same DocumentEntry (e.g. Invoice Price Variance) the unprocessedProjectCosts api throws several errors telling me to populate additional details such as: "You must provide a value for the Supplier attribute." But the API does not have any such attribute.