highest total fee

This is relatively straightforward to do with just measures and INDEX.

First, quick overview of my test data and model. Will share the M/DAX for generating these at bottom of post if interested.

I generated 10k rows of test data, Sales, in Power Query similar to below (all dates in 2024):

Sales

I made a calculated table, Dates, marking as a date table and relating to Sales.

So, my model looks like: 

It sounds like you want to display the 'Top Date' by SUM( Sales[Amount] ) and also display said amount, something equivalent to: SUM( Sales[Amount] ) where Date = 'Top Date'.

Here are measures to achieve this:

Top Date = 
VAR _topDate = 
    INDEX( 
        1, 
        SUMMARIZECOLUMNS( Dates[Date], "SalesOfDay", CALCULATE( SUM( Sales[Amount] ) ) ), 
        ORDERBY( [SalesOfDay], DESC ) 
    )
RETURN
CALCULATE( VALUES( Dates[Date] ), _topDate )

Top Amount = 
VAR _topDate = 
    INDEX( 
        1, 
        SUMMARIZECOLUMNS( Dates[Date], "SalesOfDay", CALCULATE( SUM( Sales[Amount] ) ) ), 
        ORDERBY( [SalesOfDay], DESC ) 
    )
RETURN
CALCULATE( SUM( Sales[Amount] ) , _topDate )

As you can see, the initial calculation of the 'Top Date' is handled the same for each measure using INDEX.

To showcase how these work. Here is a quick gif of the measures in cards, along with Dates[Month] slicing and a regular sorted table to validate the top values from the measures.

Code for generating tables if interested:

Sales (Power Query)

let
    Source = List.Generate( 
        ()=>0, each _ < 10000, each _ + 1, 
        each { 
            Number.Round( Number.RandomBetween( 10, 300 ), 2 ), 
            Date.From( Number.RoundDown( Number.RandomBetween( 
                Int64.From( #date(2024,1,1) ), 
                Int64.From( #date(2024,12,31) ) + 0.99999 
            ) ) )
        } 
    ),
    ToTable = Table.FromRows( Source, type table [ Amount = Currency.Type, Date = date ] )
in
    ToTable

Dates (DAX)

Dates = 
GENERATE(
    CALENDARAUTO(),
    VAR _dt = [Date]
    VAR _yr = YEAR( _dt )
    VAR _qr = QUARTER( _dt )
    VAR _moNo = MONTH( _dt )
    VAR _mo = FORMAT( _dt, "mmm" )
    RETURN
    ROW(
        "Year",_yr,
        "Quarter",_qr,
        "Month No",_moNo,
        "Month",_mo
    )
)