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 ?
from collections import defaultdict
from odoo import api, fields, models, _
from odoo.exceptions import UserError
from odoo.tools import float_compare, float_round
import logging
_logger = logging.getLogger(__name__)
class StockInventory(models.Model):
    _inherit = "stock.inventory"
    accounting_date = fields.Date(
        'Force Accounting Date',
        help="Choose the accounting date at which you want to value the stock "
             "moves created by the inventory instead of the default one (the "
             "inventory end date)")
    @api.multi
    def post_inventory(self):
        acc_inventories = self.filtered(lambda inventory: inventory.accounting_date)
        for inventory in acc_inventories:
            res = super(StockInventory, inventory.with_context(force_period_date=inventory.accounting_date)).post_inventory()
        other_inventories = self - acc_inventories
        if other_inventories:
            res = super(StockInventory, other_inventories).post_inventory()
        return res
class StockLocation(models.Model):