I must suggest that the current implementation of how the wizard recalls the deposit can be improved greatly by a simple change:
Instead of getting the net balance associated with the party from the desposit account, it should allow to select from the deposits made to/from the party. This way, one can have different deposits and relate/reconcile them to different invoices relating to the same seller/buyer.
Simultaneously, the wizard can also offer to enter an amount to credit/debit the deposit account in case the selected deposit lines don’t fully add up to pay the total invoice amount.
This method (i.e. use of account_deposit) is a simpler solution to handle cash purchases/sales.
I do not think such behavior is needed.
For the case of purchase, you must deduce the same deposit amount as the supplier did on the invoice he sent.
For the case of sale, you have two use cases:
You collect deposit without any specific purpose, so you deduce on the sale the maximum amount (what customer will expect)
You collect deposit with specific purpose, so you must create an account for each purpose so you can collect from the specific account. As creating those account may be time consuming, you can also use the sale_advance_payment which manage advance payment sale by sale.
Based on the above, the two options I have for handling cash purchases (and similar procedure for sales) are
Solution #1:
This is not technically corrrect from an accounting/book-keeping standpoint because an invoice has to be posted for the full amount before any prior deposits are deducted. The problem is that this is recorded as a credit purchase (payables), even though the deposit account may be credited immediately after posting the invoice using a reconcile write-off from the statement line.
Solution #2:
Use account_deposit. In this case, the deduction of deposit is made before posting the invoice, thereby reflecting the true situation in the accounting records/moves.
The second solution (use account_deposit) is preferable over the first solution because it shows the deposit deduction as a line on the invoice (see post #16), making clear to the reader on a single page the state of the entire transaction.
For me, the deposit recall by the wizard should also be able to ask the user to reconcile the invoice deduction with a statement line (which the user can select from a list). This would make it more convenient to track advance payments made to suppliers for specific purchases. The current method of deposit recall pays part/all of the invoice using whatever balance exists with the supplier, and can remain the default option. But the more controls one has over such transactions/adjustments, the better the usability and the lower the time consumption.