The suspect initially offered his services to Russian intelligence for financial gain. Apparently this is what alerted him to German counterintel. There was then an offer made to a representative of the USA.
But most important, Spiegel reports that it hasn't been confirmed yet that this USA representative was actually American or acting in the capacity of the government, suggesting that it may have been a counterintelligence operation.
[0] http://www.spiegel.de/politik/deutschland/bnd-mitarbeiter-so...