أموال غير شرعية

slush fund n.: A fund raised for undesignated purposes, especially: A fund raised by a group for corrupt practices, such as bribery or graft.

English term (edited): "slush funds"

Notice that the phrase in question is placed between quotes, indicating that this is a "wink-wink" term borrowed from an older usage and now generalized as a derogatory term for concepts of similar connotations.

In order to preserve this shift of tone (from the stright term "off-book accounts" to the more colorful term "slush funds"), you need to find an Arabic term with a similar connotation -- a term that is pejorative in a slangish, less formal way. Such terms exist in Arabic, but, due to the jig-saw fragmentation of contemporary Arabic slang, different terms may be used in different regions for the same purpose. Whichever term you decide to use, it needs to be placed between quotation marks to indicate the intended tone.

However, if you decide to use a "straight" term, one without a special color or flavor, you will need to remove the quotation marks. The straight term that I would suggest is

حسابات مشـبوهة

Whatever term you use, the implication of corruption needs to remain within the foggy realm of suspicion, not the realm of conviction, even though the practice of keeping such accounts is considered illegitimate. The suspicion arises from the fact that the accounts are "off the books."

Business Definition for: Slush Fund
http://dictionary.bnet.com/definition/slush fund.html