set dateformat dmy;

Declare @t table 
 (Kod varchar(10),
 date1 date,
 date2 date,
 summa decimal(10,2))

 Insert into @t 
VALUES ('EOF001','01.01.2019','28.02.2019', 1000),
       ('EPR001','01.12.2018','15.01.2019',300.01),
       ('ДляОкругл','17.12.2018','15.01.2019',300.01);



With dates as (Select (Select dateadd(d,1-day(min(date1)),min(date1)) From @t) as Date_Begin, 
                      (Select max(date2) From @t) as Date_End),

  calendar as (Select Date_Begin as Month_Begin,
                      dateadd(d,-1,dateadd(m,1,Date_Begin)) as Month_End
                 From dates
                union all
               Select dateadd(m,1,Month_Begin),
                      dateadd(d,-1,dateadd(m,2,Month_Begin))
                 From calendar 
                 join dates on dateadd(m,1,Month_Begin) <= Date_End),

 Month_Sum as (Select a.Kod,
                      iif(a.date1>c.Month_Begin,a.date1,c.Month_Begin) as Date1,
                      iif(a.date2<c.Month_End,  a.date2,c.Month_End)   as Date2,
                      Cast(a.Summa
                           *(datediff(d,iif(a.date1>c.Month_Begin,a.date1,c.Month_Begin),iif(a.date2<c.Month_End,  a.date2,c.Month_End))+1)
                           /(datediff(d,a.date1,a.date2)+1) as Decimal(10,2)) as Summa_Month,
                      a.Summa as Summa_All
                 From @t a
                join calendar c on a.date1<=c.Month_End and a.date2 >= c.Month_Begin)
--Убираем ошибку окргуления
 Select Kod,Date1,Date2,
        Summa_month
        +iif(row_number() Over (Partition by Kod,Summa_all Order by Summa_Month desc) = 1, 
             summa_all-sum(Summa_month) over (Partition by Kod,Summa_all), 
             0)
   From Month_Sum
  Order by Kod,date1