Actually it is possible to buy duty free from MAN when making an EU connection to a non-EU destination. I did this when flying Lufthansa MAN-FRA-ZAGREB I did not think it would be possible but asked anyway and was told it was not a problem as long as I showed the second boarding pass. I did and managed to buy cigarettes which are only on sale to non-eu destinations. Of course this was MAN T1 and not T3 but the same thing should be possible.