LineItem with negative amount

ARTS does not support negative amounts, this needs to be posted as RetailPriceModifier:
 

<LineItem>
  <Sale ItemType="Stock">
    <ItemID/>
    <RetailPriceModifier>
      <SequenceNumber>1</SequenceNumber>
      <Amount Action="Subtract">10.00</Amount>
      <Description>Felslagning</Description>
    </RetailPriceModifier>
    <Tax TaxType="VAT">
      <TaxableAmount>8.0</TaxableAmount>
      <Amount>2.0</Amount>
      <Percent>25</Percent>
    </Tax>
  </Sale>
  <SequenceNumber>1</SequenceNumber>
</LineItem>