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 ?
"""
        Generate the account.move.line values to post to track the stock valuation difference due to the
        processing of the given quant.
        """
        self.ensure_one()
        if self._context.get('force_valuation_amount'):
            valuation_amount = self._context.get('force_valuation_amount')
        else:
            if self.product_id.cost_method == 'average':
                valuation_amount = cost if self.location_id.usage == 'supplier' and self.location_dest_id.usage == 'internal' else self.product_id.standard_price
            else:
                valuation_amount = cost if self.product_id.cost_method == 'real' else self.product_id.standard_price
        # the standard_price of the product may be in another decimal precision, or not compatible with the coinage of
        # the company currency... so we need to use round() before creating the accounting entries.
        debit_value = self.company_id.currency_id.round(valuation_amount * qty)
        # check that all data is correct
        if self.company_id.currency_id.is_zero(debit_value):
            if self.product_id.cost_method == 'standard':