Can't give 5/10 Mark banknotes!!!

Even I press the key combination for 5/10 mark banknotes, even I set my own bind which is only 1 key,then still I cannot give the fares 5DM exchange!!!
I drive SD200 1980. I strongly think there must be 5/10Marks in that bus if it is not scripted...Please hhelp.

You can't. You can only give coins back, since the highest note is the DM 10:- and DM5:-.

See manual caapter 4.3.3 Ticket sale.