To create a supplier invoice, go to Deals List. Select the Deal for which you need to create the supplier invoice then click on edit symbol and chose Create Supplier Invoice.
  

A new window opens that contains the arrangements of the Deal. If you haven't set your suppliers for the arrangements displayed in the list you can edit them at this point and select a supplier for each one. Also, you can also select a Buy Price in case this was not set already per product price.

After you have set up the supplier and saved the changes, select the arrangement to be invoiced. To create the invoice you must write the Reference invoice code in the dedicated field and Reference invoice total amount must be equal with the Total amount.

GO TO OVERVIEW and if the invoice is final, mark the checkbox for IS FINAL INVOICE. That means that no other invoice will be created to this supplier for the current reservation and arrangement.

Keep in mind that for an already created supplier invoice exists only the possibility to edit whether the invoice is final or not. Other details cannot be edited. If the invoice is not correct regarding the other details you must do a credit note to that invoice and redo the invoice with the correct details.