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
''' This function is called at the end of negative quant reconciliation
        and does the accounting entries adjustemnts and the update of the product
        cost price if needed '''
        super(StockQuant, self)._price_update(newprice)
        for quant in self:
            move = quant._get_latest_move()
            valuation_update = newprice - quant.cost
            
            
            if not quant.company_id.currency_id.is_zero(valuation_update) and move._check_lock_date():
                quant.with_context(force_valuation_amount=valuation_update)._account_entry_move(move)
            
            
            
            
            if quant.product_id.cost_method == 'real' and quant.location_id.usage != 'internal':
                move._store_average_cost_price()
    def _account_entry_move(self, move):