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External purchase
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The external purchase object describes an external purchase that can be part of an order.

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Field name         Type                  Description
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title              :doc:`i18n`           The title of the external purchase to be printed on the invoice.
product_reference  String                A reference to a product for an external purchase.
unit_price         :doc:`amount`         The price of a unit that comprise an external purchase.
quantity           Number                The number of units that comprise an external purchase.
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