Use this code to set remote file permissions via SFTP from PowerShell:
$sftp = New-Object Rebex.Net.Sftp
$sftp.Connect("server")
$sftp.Login("user", "password")
$attributes = New-Object Rebex.Net.SftpAttributes
$attributes.Permissions = [Rebex.Net.SftpPermissions] "OwnerWrite, OwnerRead"
$sftp.SetAttributes("file.txt", $attributes)
$sftp.Disconnect()
Learn to use Rebex SFTP and other components from PowerShell.