Tough one. Closest I can find is declining-balance basis / system / plan.

German "degressive Abschreibung" is "declining-balance depreciation method".

[PDF]Vorlesung 2
... Abschreibung ***Declining Balance Method degressive Abschreibung *** Sum of ...
www.tu-dresden.de/wwbwlwus/download/ ws0102/vorlesung30.10.01.pdf -

Lexikon Buchstabe D -... ***declining balance depreciation***, Wirtschaft, degressive Abschreibung. home.t-online.de/home/NDickmeis/lexd.htm - 101k


Textron responded on May
28 with a "declining balance" proposal. Under this plan, the
employees laid off earliest would receive the greatest sever-
ance pay; the pay would gradually decrease over the period
of the agreement, declining to zero benefits upon expiration of
the EBA.

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There is also a declining balance co-ownership plan for homes, but it doesn't seem very applicable here. It is for rules of Sharia and not paying interest. If you are interested, here is the link:
http://www.guidancefinancialgroup.com/products/OurProgram.as...

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Degressive is, of course, an English word, usually having to do with taxes:
From RH: degressive adj. - pertaining to a form of taxation in which the rate diminishes gradually on sums below a certain fixed amount.

Also not entirely unrelated, obviously, but I can't find anything that remotely fits with degressive. "degressive" in google gets you a few English and mostly German or French websites.
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Hope that helps...