API - Customer and Invoice Custom fields and posting payments

API - Customer and Invoice Custom fields and posting payments

Custom Fields-
Is there an API method to retrieve the 10 custom fields in the customer and invoice tables?
These can be upload via CSV import, but I did not see a way to access them GET or POST via the API.
[This is separate from the 3 custom fields for the customer that are part of the API Customer Detail]

Payments-
Within the UI you can create a credit by entering a payment and not posting the full amount against an invoice, but I could not find an API method to create a credit. POST ing a payment against an invoice works fine,but is limited to the invoice total.  We frequently have overpayments and issue credits that we would like to automated via the API.

If there are not methods that currently support this functionality, can you advise if it is on the project timeline?