Monthly Budget to Working Days

I don't think you fully understand the proposed solution. You have Budget data at a MONTHLY grain like this:

Month       Budget      Working Days

====        =====     =========

1               22,000        22

2               21,000        21

And you have Dates like this:

Month     Date    Working Day

====      ====  ========

1             1/1       0

1             1/2       1

1             1/3       1

You need to JOIN the two tables on Month (and Year):

Month     Budget     Date     Working Day      Working Days

=====    =====   ====   =======         =========

1               22,000     1/1       0                         22

1               22,000     1/2       1                         22

...

2               21,000     2/1       1                         21

Now create a calculated column in this table:

[Daily Budget] = [Monthly Budget] / [Working Days] * [Working Day]

You don't need SUM here! If using this sample data, the [Daily Budget should come out to 1,000 per working day, and 0 for non-working days.

Now the MTD calculation uses the SUM function:

[MTD Budget] = TOTALMTD ( SUM ( [Daily Budget] ), 'Date Dimension'[Date] ).