Create field to get record from XML wizard invoice in

Scenery: We have odoo 12v community, a custom module to invoice under the regulations of the Chilean tax service that inherits the functionality of the native account module. We have the problem when a supplier invoice is received through XML (cusmto module have wizard), some supplier XML in the invoice line contains a "discount" field, said field contains the amount of the discount discount, when the invoice enters the system through fetchmail or is imported from the wizard, it is not possible to know the value of the cost of the product with the discount on the invoice line in the system view. for example this is a linea invoice: <Detalle> <NroLinDet>15</NroLinDet> <CdgItem> <TpoCodigo>INTERNA</TpoCodigo> <VlrCodigo>102122</VlrCodigo> </CdgItem> <NmbItem>RONSON ENCENDEDOR TRANSPARENTE-1UDX20</NmbItem> <DscItem>ODD: 000000574 SubOdd: 00015 Cant.: 3.000,00 Asignado : 3.000,00</DscItem> <QtyItem>3000</QtyItem> <UnmdItem>UN</UnmdItem> <PrcItem>171.05</PrcItem> <DescuentoMonto>51315</DescuentoMonto> <MontoItem>461835</MontoItem> </Detalle> where "<DescuentoMonto> is a field that not do calculate and not see on system. request: 1.- add field to linea invoice from new custom module to create 2.- make a calculation that balances the amounts with the visible fixed-type discounts Considerations: 1.- We have module custom with wizard 2.- do testing before of delivery us 3.- deliverables on file .zip module custom