Toggle between absolute values and % - using field parameters

If you have access to the model you could create a static table via 'Enter Data'.

Lets call the table 'Display Mask'

Add a column and name it 'Display'

Add a value 'Absolute'

Add another value '%'

Save and exit power query to apply the changes.

You should have 2 measures already created (if not you should do this first)

1 measure will hold the Absolute value

the other measure in %

You will then create a new measure, lets call it 'User Preference' and define it as below.

User Preference = 

VAR _SelectedDisplay = SELECTEDVALUE('Display Mask'[Display],"Absolute") (Default selection will be abs if nothing selected)

VAR _Abs = [Absolute Value]

VAR _% = [% Value]

RETURN

SWITCH(

TRUE(),

_SelectedDisplay = "Absolute", _Abs,

_SelectedDisplay = "%", _%

)

N.B Be sure to drop the column from your new table into your report as a slicer