Enable incoming invoices to be linked to outgoing invoices

[timo2o1o]

Is your feature request related to a problem? Please describe.
As an accounting guy I want to be able to link incoming invoice positions to outgoing invoice positions. This way I can make sure, that all invoices issued to me are billed to the customer.

Describe the solution you'd like
The Ausgangsrechnungsposition allready allows to link a list of Eingangsrechnungspositionen. But there needs to be a possibility to assign them.