Clear short term file lock of a file in a SharePoint 2013+ document library when the file is still open by the user. Replace url, list (List Title) and item ID.

SP_clearShortTermFileLockImpersonation.ps1

$web = get-spweb "https://domain/sitecollection/site"
$list = $web.Lists["Documents"]
$item = $list.Items.GetItemById(123)

$userId = $item.File.LockedByUser.ID
$user = $web.AllUsers.GetById($userId)

$impSite = New-Object Microsoft.SharePoint.SPSite($web.Url, $user.UserToken);
$impWeb = $impSite.OpenWeb();
$impList = $impWeb.Lists[$list.Title];
$impItem = $impList.GetItemById($item.ID);
$impItem.File.ReleaseLock($impItem.File.LockId)

$impWeb.Dispose()
$impSite.Dispose()
$web.Dispose()