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 "Außenhandels-Einnahmen-Klausel", 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 "Außenhandels-Einnahmen-Klausel" (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?