Recurring payment on custom gateway

Hi,

We use WHMCS for internal Web Hosting management and I've got a special use case I can't figure out.
As all the payments are handled through our PeopleSoft system I have setup a custom gateway through which clients enter their internal account number. After that we setup the payments in PeopleSoft manually, both one time and recurring. And the problem is with the recurring payment: as there is no link between WHMCS and PeopleSoft, WHMCS has no way of knowing that the recurring monthly payment are really done, so invoices are generated and left unpaid.
I see three solutions:
- Develop something in PeopleSoft that sends a payment to WHMCS when it's done. Not so easy.
- Have WHMCS stop generating invoices after the first one. I don't like it too much as I want clients to be reminded they still have something going on. And this would also break the revenue dashboards...
- Have WHMCS generate automatically a payment when it creates the invoice. For that I will have to reuse the account number that was provided with the first payment as I use it for reference in the transaction ID. That would be my preferred solution, but how to do that, more precisely:
* how to collect the internal account number provided with the payment? Well collecting is easy, I do it for my transaction ID, but where to store it associated with a specific command (yes, clients can provide many different accounts depending on what they buy)?
* how to reuse it afterwards when the invoice is generated?
I know there must be some way because it works almost like this for recurring CC payments.

Thanks a lot for your help if you have any idea...

Guillaume.