Impacted versions:
9.0

Steps to reproduce:
- Create a product with costing method set to average
- Create a purchase order for the product in a foreign currency, and validate
- Change the exchange rate for the currency in question
- Receive the product

Current behavior:
- Stock quants get the correct cost behind the scene (PO value at current rate)
- Accounting journals get the correct value
- Stock move price unit has been updated to the correct value
- **Average cost on product file is the PO value at old rate**

Expected behavior:
- Average should have been updated in line with the new rate

Video/Screenshot link (optional):

cacc = accounts['expense'].id
            if dacc and cacc:
                price_unit = i_line._get_anglo_saxon_price_unit()
                return [
                    {
                        'type':'src',
                        'name': i_line.name[:64],
                        'price_unit': price_unit,
                        'quantity': i_line.quantity,
                        'price': self.env['account.invoice.line']._get_price(inv, company_currency, i_line, price_unit),
                        'account_id':dacc,
                        'product_id':i_line.product_id.id,
                        'uom_id':i_line.uom_id.id,
                        'account_analytic_id': False,
                    },
                    {
                        'type':'src',
                        'name': i_line.name[:64],
                        'price_unit': price_unit,
                        'quantity': i_line.quantity,
                        'price': -1 * self.env['account.invoice.line']._get_price(inv, company_currency, i_line, price_unit),
                        'account_id':cacc,
                        'product_id':i_line.product_id.id,
                        'uom_id':i_line.uom_id.id,
                        'account_analytic_id': False,
                    },
                ]
        return []
    def _prepare_refund(self, cr, uid, invoice, date_invoice=None, date=None, description=None, journal_id=None, context=None):