You can explicitly set a property value rather than have the property derivation logic supply one, by specifying an override-property attribute in the <derivation> tag:

<item-descriptor name="employee">
  <property name="department" item-type="department"/>
  <property name="empSpendingLimit" data-type="int"/>
  <property name="spendingLimit">
    <derivation override-property="empSpendingLimit">
      <expression>department.spendingLimit</expression>
    </derivation>
  </property>
</item-descriptor>

The derivation tag here specifies that if the empSpendingLimit property is not null, it is used as the value of the spendingLimit property, otherwise the spendingLimit property is derived as before. The override-property attribute lets you edit in a Repository Editor a property that is otherwise derived from another property.