Unfortunately recurring invoices always create a new invoice document with a fresh invoice number in the journal. And this is exactly the thing we are trying to avoid. We imagine a special invoice type that periodically increases the debit of the partner, but for obvious (legal) reasons without changing the invoice amount itself. And if the debit of the partner increases every month (without having to create new invoices) we would had something the reconcile the payments against. Probably we could summarize that as follows: Is there a clever way to increases the partner's debit periodically by a certain amount? However there should still be a connection to the original invoice.

I do not really understand why you try to avoid new invoice numbers. In the overall process view including dunning management it is the easiest thing to do and it's automatically. For convenience purposes you could consider to build a model to create customer/contract sequence number, see this discussion: https://www.odoo.com/forum/help-1/question/changing-invoice-sequence-numbering-to-include-customer-reference-17213

Believe me, I personally would really prefer a simple out-of-the-box solution like the automatically created recurring invoices as well. But the management of my company strongly vetoes against that approach. They say it is problematic since we are talking about foreign trade business. For example we have to submit every single invoice to the bank in order to release the received payment (thank you, German law!). With some kind of "permanent, periodic invoice" we would have to do that only once for each contract, if we generate a new invoice every month we would have to do that every month for each contract. [Disclaimer: I don't know whether this factually correct, I'm not a legal expert, I'm just the poor software developer who shall implement it]

Ah, I see (but still do not really understand, but who understands banks anyway). Just as an idea, because the bank seems not to understand the term subscription contract, why don't you create a contract template which is titled something like "(Permanent) Invoice" instead of "Contract"? So your banks sees an "invoice" and you can use the standard recurring invoices facility in order to properly manage the sales and payment process.

The customer only wants to get one invoice with one invoice number... so all monthly payments will show that one invoice number... and will not match the monthly generated invoice numbers in Odoo, Basically the customer gets a contract that explains that he has to pay a monhtly subscription fee. The customer does not want to get a monthly invoice, because depending on the company the accounting department will have to get a new permission every month to pay the invoice, because it is a new invoice number and new invoice every month. With the subscription approch there is only one contract and invoice that tells the customer to make monthly payments until the customer decides to cancel the subscription.

You do not send these monthly invoices to the customer nor to the bank. You just keep them for your internal reconciliation of the payments.

Thanks for the hints. Now my problem is to convince our management of this approach. Surely this will be a tough job as they are really sceptical. But I'll try...

As far as I could see the "Dauerrechnung" can be questionable, see http://www.lexware-verbindet.de/index.php?action=printpage;topic=6118. And it is not more efficient from an administrative or software-technical point of view. It is a fact, the business case is the case of contract-based recurring invoice, and that's the way, most software is built for. Anything else might be something from the times, when invoices were written with a typewriter :-)

This is indeed a very good question. I guess such invoices with periodic payments would remain always open, unless the according contract is finally terminated. Maybe these special invoices would need an additional state for "all fees paid, waiting for next monthly fee".