This is not possible natively within Core. However, there are a few ways you can try to invoice an expense partially (now some and later some):
- Create an invoice for the full value with a memo giving instructions for partial payment. Leave it open until it's fully paid.
- Create retainer for the percentage of the full value that you need up front, then include an expense report when sending it to your client.
- Create an invoice at 100% value with a retainage of 50%. The client pays 50% immediately and pays the other 50% later.
- Create an invoice at 50% value. Collect the payment for this invoice and store the money received in a retainer. Delete the invoice and create a new invoice for 100% value later. Use the retainer and the client's new payment to pay off the second invoice.