Escludi determinati valori quando dividi un valore fisso in paesi in base a una quota di compartecipazione alle entrate giornaliera

Puoi usare case espressioni, entrambe intorno il calcolo e entro la finestra sum :

select 
    sales_date, 
    country, 
    sum(sales_volume),
    case when country <> 'NL'
        then sum(sum(fix_costs)) over(partition by year(sales_date), month(sales_date))
            / day(last_day(sales_date)) 
            * sum(sales_volume)
            / sum(case when country <> 'NL' then sum(sales_volume) else 0 end) over(partition by sales_date)
    else 0
    end as fix_cost_per_day
from sales
group by 1,2;

Demo su DB Fiddle :

sales_date | country | sum(sales_volume) | fix_cost_per_day
:--------- | :------ | ----------------: | ---------------:
2020-01-03 | DE      |               500 |      37.95066414
2020-01-03 | FR      |               350 |      26.56546490
2020-01-03 | NL      |               320 |             null
2020-01-30 | None    |                 0 |             null
2020-02-15 | DE      |               700 |     137.14733542
2020-02-15 | FR      |               180 |      35.26645768
2020-02-15 | NL      |               420 |             null
2020-02-29 | None    |                 0 |             null
2020-03-27 | DE      |               180 |      20.19635344
2020-03-27 | FR      |               970 |     108.83590463
2020-03-27 | NL      |               670 |             null
2020-03-31 | None    |                 0 |             null