> Should we put marketplace.owner_customer.debitable_bank_accounts[0] then?

No, `source_uri` _is_ taken from that collection already so if it is null then that collection must also be empty.

> Will PUT source_uri to /invoices/<guid> work?

Yes, but only for that particular invoice. Which is of little value given you have a small window to do this and it doesn't effect any future invoices.