Variable property of Powershell function output does not work as expected

I'm using Get-Diskfree function described here: https://binarynature.blogspot.com/2010/04/powershell-version-of-df-command.html

I want to sum all values from Used column, but as I try to call out a specific properties of Get-Diskfree output, I've got nothing, null.

Function is showing output like this:

FileSystem : NTFS
Type       : Local Fixed Disk
Volume     : C:
Available  : somevalue
Computer   : somevalue
Used       : somevalue
Size       : somevalue

FileSystem : NTFS
Type       : Local Fixed Disk
Volume     : D:
Available  : somevalue
Computer   : somevalue
Used       : somevalue
Size       : somevalue

I want to be able to call out Used property by parsing Get-Diskfree output into variable like this:

function {...}
$variable = Get-Diskfree
$variable.Used

Last line don't give me any output, in comparision to for example:

$variable = get-service
$variable.Name
long list of service names

Is function (or functions overall) designed in a way, that this method is impossible to use?

I plan to sum them by using more less the method I used in this script Powershell: System.Object[] in export-csv file

What I want to achieve is to sum Used column for first 8 disks (there are 20 of them on VM), export an output as csv and send an html email based on that csv file.