The landed costs button_validate() method attempts to create a correction in quant cost that occurs due to currency rounding. However this can lead to a negative cost value being assigned to a stock.quant

The issue affects move lines with a very low price to quantity ratio. ie 1000 x $0.10
Applying a landed cost of $456.70 to this line gives a diff of $0.4567
Having a 2 decimal rounding on the currency makes diff_rounded = $0.46

Now the formula used to determine the cost correction (diff_correct) on the single qty quant (quant_correct)
```python
diff_correct += (line.quantity * diff) - (line.quantity * diff_rounded)
```
This equates to diff_correct = 0.46 + (1000 * 0.4567) - (1000 * 0.46) = -2.84

and then
```python
if quant_correct:
    quant_dict[quant_correct] = quant_correct.cost + diff_correct
for quant, value in quant_dict.items():
    quant.sudo().write({'cost': value})
```
The correction quant's cost becomes $-2.74

If this quant is then in an inventory adjustment, the accounting lines generated violate a constraint on the journal that (credit + debit) >= (0)

I would suggest that the solution is that this line should be changed to only ever round down
```python
diff_rounded = tools.float_round(diff, precision_rounding=curr_rounding)
```
If rounded down in this case, diff_rounded = 0.45  and diff_correct would be calculated as $7.15 and quant.cost = $7.25
A disproportionately large cost for a $0.10 product, but at least it's positive.

Perhaps a rounding_method='DOWN' should be implemented in tools.float_utils ?
addons/stock_account/models/stock.py
'product_uom_id': self.product_id.uom_id.id,
            'ref': self.picking_id.name,
            'partner_id': partner_id,
            'credit': credit_value,
            'debit': 0,
            'account_id': credit_account_id,
        }
        res = [(0, 0, debit_line_vals), (0, 0, credit_line_vals)]
        if credit_value != debit_value:
            # for supp
            diff_amount = debit_value - credit_value
            price_diff_account = self.product_id.property_account_creditor_price_difference
            if not price_diff_account:
                price_diff_account = self.product_id.categ_id.property_account_creditor_price_difference_categ
            if not price_diff_account:
                raise UserError(_('Configuration error. Please configure the price difference account on the product or its category to process this operation.'))
            price_diff_line = {
                'name': self.name,
                'product_id': self.product_id.id,
                'quantity': qty,
                'product_uom_id': self.product_id.uom_id.id,
                'ref': self.picking_id.name,
                'partner_id': partner_id,
                'credit': diff_amount > 0 and diff_amount or 0,
                'debit': diff_amount < 0 and -diff_amount or 0,
                'account_id': price_diff_account.id,
            }
            res.append((0, 0, price_diff_line))
        return res