BERLIN (Reuters) - The German government last year posted its biggest surplus since reunification in 1990, the Finance Ministry said on Monday, adding that this was partly due to interest payments being lower.A fund set aside for helping to integrate migrants now contains 48.2 billion euros ($53.61 billion), of which around two-thirds have already been earmarked for use, the ministry said. It said there were around 17.1 billion euros in that fund still available for use.