Do German Goverment bonds usually include (or included) a condition that the debt will not be serviced in case of trade imbalance?
The answer is somewhat. Those terms were not part of the original bond but were added by the Debt Agreement


  The Conference recognised the principle that the transfer of payments under the Settlement Plan implies the development and maintenance of a balance of payments
  situation in which those payments, like other payments for current transactions, can be financed by foreign exchange receipts from visible and invisible transactions so
  that more than a temporary drawing on monetary reserves is avoided. (Final Report of the Conference on German External Debt 1952: paragraph 21)


That quote is NOT part of the official agreement (As seen by it's absence in the British  and the Swiss copy) - however the "One made it out of the debt trap" study claims it was accepted by the British and the French.