[FIX] account: keep write off sign consistent

Create a payment like follow:
- Payment type: receive money
- Partner type: customer
- Destination account: whatever receivable
- Amount: define an amount (i.e. 90.00)
On Save, open the draft jounral entry created.
Lower the liquidity or counterpart amount and add a writeoff
Go back to the payment, change the amount and save.

The journal entry writeoff will swap from debit to credit at every
change made to the amount. This occur also when changing currency on the
payment

opw-2475223

closes odoo/odoo#70303

Signed-off-by: Laurent Smet <smetl@users.noreply.github.com>