Confirm save command

Hello,

I have this script:

cls
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.Client")  
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.Build.Client")  
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.Build.Common") 
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.WorkItemTracking.Client") 
$tfsCollectionUrl = "http://zdev:8080/tfs/dev_test_collection"
$teamProjectCollection = [Microsoft.TeamFoundation.Client.TfsTeamProjectCollectionFactory]::GetTeamProjectCollection($tfsCollectionUrl)
Write-Host "Team Project Collection: "$teamProjectCollection
$ws = $teamProjectCollection.GetService([type]"Microsoft.TeamFoundation.WorkItemTracking.Client.WorkItemStore")
write-host "WorkItemStore: "$ws
$buildServer = $teamProjectCollection.GetService([Microsoft.TeamFoundation.Build.Client.IBuildServer])
Write-host "Build Server: "$buildServer
$buildDetail = $buildServer.QueryBuilds("ms-basis dev", "Staging_Test_DEV") |  `
	where { $_.BuildNumber -eq "Staging_Test_DEV_20130604.1" }
Write-Host "Buildnumber: "$buildDetail.BuildNumber
Write-Host "Drop Location: "$buildDetail.DropLocation
Write-Host "Retain Indefinitely bevore: "$buildDetail.KeepForever
$buildDetail.KeepForever = "false"
Write-Host "Retain Indefinitely after: "$buildDetail.KeepForever
$buildDetail.Save()

I want to change the Retain Indefinitely with this script to false. But the script doesn't change the value.

When I change this Retain Indefinitely to false with Visual Studio, I have to confirm this action.

How can i confirm this action automatically with at powershell.

This $buildDetail.Save() -confirm:$true does not work.

The other way, change the Build Retain Indefinitely from false to true works with this script!

I hope somebody can help me,

Thanks Horst MOSS 2007 Farm; MOSS 2010 Farm; TFS 2010; IIS 7.5