Completing German master's programs in less than allotted time I'm considering applying to master's programs in mathematics over in Germany (e.g. Bonn, Berlin Mathematical School). The stated time for these programs is two years. 

Should I get accepted and attend, are there any issues (ethical or otherwise), with bailing after a year to attend a PhD program in the US? If so, is it possible to complete the program in a single year if I come in with some kind of advanced standing (i.e. having taken many graduate courses beforehand)?
This is something that you have to ask the respective offices at the schools you apply.

You can savely assume that completing a Master's in Germany in less than two years is in general possible if you complete all required things in time, i.e. you've got enough credits in all required categories. It will not hurt if you double check with the "Studien-" or "Prüfungsordnung" of the respective programs.

If the program will award credits for courses that you have taken somewhere else is something that nobody but the program office can answer (and will probably finally decided on an individual basis by the dean of student affairs).