Do German Goverment bonds usually include (or included) a condition that the debt will not be serviced in case of trade imbalance? Today the Spiegel ran an article on a proposal to end the Euro Debt Crisis.
That article made the claim given in the title

Syriza will eine sogenannte Wachstumsklausel für jene Schulden einführen, die nach dem Schuldenschnitt übrigbleiben. Im Falle Deutschlands handelte es sich damals um eine &quot;Außenhandels-Einnahmen-Klausel&quot;, der zufolge der jährliche Schuldendienst drei Prozent der Exporteinnahmen nicht übersteigen durfte. Für die Gläubiger Deutschlands hieß es also: Kauft deutsche Produkte, wenn ihr euer Geld zurückwollt.
Syriza wants to establish a so called growth-clause for the debt left after the haircut. In the case of Germany there was the &quot;Außenhandels-Einnahmen-Klausel&quot; (Foreign-trade profit condition) which states that the amount of serviced debt is not allowed to be higher than 103% of export profit.
For the debtee that meant: If you want your money back you have to buy German products.

As the article mentions the London Debt Agreement of 1953 I assume that the statments given in the past tense refer to that agreement. However the term Außenhandels-Einnahmen-Klausel does not appear in those documents and on a cursory overview actually doesn't seem to include those conditions.
The only google hits for that exact term are that very article.
And the proposal also doesn't mentions foreign trade or export.
So, has this clause existed or even still exists today?
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.