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 ?
self.product_price_update_before_done()
        res = super(StockMove, self).action_done()
        self.product_price_update_after_done()
        return res
    @api.multi
    def product_price_update_before_done(self):
        tmpl_dict = defaultdict(lambda: 0.0)
        
        std_price_update = {}
        for move in self.filtered(lambda move: move.location_id.usage in ('supplier', 'production') and move.product_id.cost_method == 'average'):
            product_tot_qty_available = move.product_id.qty_available + tmpl_dict[move.product_id.id]
            
            if product_tot_qty_available <= 0:
                new_std_price = move.get_price_unit()
            else:
                
                amount_unit = std_price_update.get((move.company_id.id, move.product_id.id)) or move.product_id.standard_price
                new_std_price = ((amount_unit * product_tot_qty_available) + (move.get_price_unit() * move.product_qty)) / (product_tot_qty_available + move.product_qty)
            tmpl_dict[move.product_id.id] += move.product_qty