How did Hans avoid meeting Death in World War 1? What is the debt that Hans carries and how it directly affects Max?

Hans' Sergeant asked if anyone had good handwriting just before going into battle. Nobody wanted to volunteer, because it meant you were admitting that you'd prefer to stay behind while friends went into battle. Erik volunteered Hans, and so Hans stayed behind writing letters while the rest went out to die.

Max is Erik's son.

Erik Hans's accordion teacher and friend volunteers Hans for one of the mystery jobs (this one is writing letters) so he stays back while the battalion is wiped out. Hans owes his life to his friend Erik for taking his place that day.