I was willing to ask if the type of this field is accurate? Shouldn't it be of type currency-precise?
InvoiceItem.amount field is of type currency-precise. E.g if the adjustment is of type SALES_TAX the value in OrderAdjustmentBilling.amount field might be wrong. The problem can be noticed while using calculateInvoicedAdjustmentTotal service. Regards, Rashko Rejmer