[WHH]

I work with many companies that transfer invoices manually.

I do not see this option in the options that are there. If I then want to manually mark the invoice as paid, I get the message that I have to enter a payment ID.

When I do this, I get the error message: You must set the payment gateway in the invoice management tab before you mark it as paid. (9999)

So I am doing something wrong somewhere, but I cannot figure out where this is.

Does anyone have any idea how to solve this so that a manual transfer is also seen as a valid payment method.

[BelleNottelling]

For tracking purposes, FOSSBilling wants to be able to create a record of a transaction on a payment gateway when you use the "mark as paid" option.

The payment ID is arbitrary and only exists for your reference so you can put whatever you want there.
When it's telling you to select a payment gateway, it's expecting you to select a payment method from the invoice's "manage" tab. It simply uses this for the record that it creates