Here's my use case:
Users make a document submission via a separate third party system. The document is manually assessed and if suitable, is accepted. The user then needs to make a payment, but they need an invoice in order to do that.
I'd like them to use CiviCRM to make the payment online. In order to send an invoice from Civi I need to have a contribution record already in place, so if I create this record and give it a pending status, how can the user's subsequent payment be logged against this record, rather than a new record being created when they submit the contribution page?