purchase_advance_payment is unusable when different accounts required for advance and final payments

[peterromao]

Is your feature request related to a problem?
In certain countries with continental accounting the advance payment account is different from the vendor account in the chart of accounts. In these countries the account payable account (that can be defined in the Invoicing /Accounting notebook page of the partner form view) is a current liabilities account and the advance payment account for the same partner is a current assets account (for which there's no provision to define in a partner view form).
As it stands, when creating an advance payment for a vendor the wizzard assumes that the payment is to be made to the vendor's accounts payable account.
A user can circumvent this by going into the payment's journal entry before confirmation and change the account to the correct one but then the link between the purchase order is lost and there's no way to use this advance payment to balance the purchase when the bill is finally issued.

Describe the solution you'd like
What users on continental accounting countries would need is a way to have odoo publish the advance payment to the journal into the correct advance payments account (current asset to be debited while bank account credited).

This account should be defined at the same time and in the same place as when defining the accounts payables and receivables. This payment should reman linked to the correct commercial partner.

When the bill is prepared the advance payment should be balanced with a transfer of funds from this account to the accounts payable account.

Describe alternatives you've considered
The alternative is to simply not use this module and register the advance payment and credit note directly in the journals manually and have no business logic helping the user manage these advance payments.

Additional context
As for the brother module for sales workflows advance payments from customers require even more legally binding actions in continental accounting. An Invoice has to be issued upon the customer's payment and when the services/goods are finally delivered they can be definitely invoiced with the correct bill based on the purchase order and a credit note to fully balance the advance payment invoice.