Refund excess balances

[rorepo]

This is for a preliminary review - have a few issues and questions.

Got things working with my credentials, with tests/local.env and after adding the custom field participant_id in settings. 'Working' only in the sense that I get success and authorized, though. When I tried checking on the dashboard, things are less clear - all transactions are shown as 'sale' and some say 'voided', 'submitted for settlement', 'gateway rejected', etc.
The transaction summary page shows credit values as 0.

I've got the functionality worked out, but am facing the following issues:

1. A rather quirky problem. The new method test_refund_of_excess_balance that I added to test_billing_payday.py works as expected, but not along with the existing method test_payday_preserves_due_until_charged. If both methods run, the credit part of test_refund_of_excess_balance fails with an authorisation error. I see a small possibility that this behaviour may be something to do with my local environment.
2. I managed to get things running with my own Braintree credentials, using tests/local.env. I get 'success' and 'authorized' responses. However, things do not look ok when checking on the Braintree dashboard - credit in the transaction summary is always 0.

In addition, I have the following questions:

1. For the moment, I'm just looking for balances > 0 to do refunds. I believe I will need to apply some other conditions e.g. to exclude 1.0 balances?
2. I've uploaded changed test fixtures as well, but since I'm now working with my own Braintree credentials, will these be relevant?

[chadwhitacre]

I believe I will need to apply some other conditions e.g. to exclude 1.0 balances?

Yeah, this gets complicated. See #3763 (comment).

I've uploaded changed test fixtures as well, but since I'm now working with my own Braintree credentials, will these be relevant?

My expectation has been that we'll handle #3763 and #3539 with one-off scripts. I'm not sure what general-purpose long-lived code changes will be necessary for those.

[chadwhitacre]

Closing post-#3810.

!m @rorepo