A literal translation is "slice of depreciation". When equipment is leased, either the owner (lessor) uses the depreciation for tax reporting, or they allow the lessee to virtually own the equipment and claim the depreciation.

With the later, the lessee has received a reduction or discount, i.e. "rondelle".



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Note added at 3 hrs (2012-05-08 19:34:32 GMT)
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Actually, it isn't clear exactly which type of lease they are referring to. Non-tax Lease is an official IRS term and doesn't necessarily apply here unless you are translating this for the IRS! I cannot find any official term for this under French law, which may explain what appears to be a substitute phrase.

A better fit would be "under the form of a lease that allows tax depreciation".

Here is a better link:
http://www.wfw.com/Publications/Publication491/$FILE/WFW%20Briefing%20-%20French%20Tax%20Leasing%202008.pdf


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Note added at 4 hrs (2012-05-08 20:12:33 GMT)
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Thanks for the link.
They appear to be referring to Article 39 C, as mentioned in the www.wfw.com link above.
Article 39 C can be found here: http://droit-finances.commentcamarche.net/legifrance/74-code...
In which case the answer is then:
"with provisions that allow tax depreciation".