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 ?
""" Accounting Valuation Entries """
        if move.product_id.type != 'product' or move.product_id.valuation != 'real_time':
            
            return False
        if any(quant.owner_id or quant.qty <= 0 for quant in self):
            
            
            
            
            return False
        location_from = move.location_id
        location_to = self[0].location_id  
        company_from = location_from.usage == 'internal' and location_from.company_id or False
        company_to = location_to and (location_to.usage == 'internal') and location_to.company_id or False
        
        
        if company_to and (move.location_id.usage not in ('internal', 'transit') and move.location_dest_id.usage == 'internal' or company_from != company_to):
            journal_id, acc_src, acc_dest, acc_valuation = move._get_accounting_data_for_valuation()
            if location_from and location_from.usage == 'customer':  
                self.with_context(force_company=company_to.id)._create_account_move_line(move, acc_dest, acc_valuation, journal_id)