In the prison, what would Helmuth trade for his soup?

Helmuth has been sent to prison, where he awaits execution for his activities against the Nazi regime. As we can imagine, the food's pretty lousy, and Helmuth has to make do with the occasional bowl of watery cabbage soup. It's possible to eat it, just as long as he doesn't actually smell it. It's certainly not a patch on the hearty beef and barley broth his grandmother used to make for him.
Although his meager rations will keep Helmuth alive in the run-up to his execution, he'd happily a exchange a bowl of soup for a letter from home. He misses his family terribly, so much so that he aches. He knows that though his actions were deeply courageous they also caused great suffering and hardship to his family, and that's something he deeply regrets.