Русско-англо-русский юридический словарь

A slush fund is a fund or account used for miscellaneous income and expenses, particularly when these are corrupt or illegal. Such funds may be kept hidden and maintained separately from money that is used for legitimate purposes. Slush funds may be employed by government or corporate officials in efforts to pay influential people discreetly in return for preferential treatment, advance information (such as non-public information in financial transactions), and other services. (Wikipedia)

In more sinister cases, a slush fund may be used as a so-called “black fund,” which is unaccounted for and kept off the books. (Investopedia)

slush fund: an amount of money that is kept for dishonest or illegal activities in politics or business (Cambridge Advanced Learner's Dictionary & Thesaurus)

slush fund: money that is kept by a business or other organization for illegal purposes (Cambridge Business English Dictionary)

They showed $2.9bn (£2bn) had been moved into Western Europe by some of the Azeri elite, while UK shell companies - largely existing only on paper - hid the true sources of the cash. Some of it was used as a slush fund to bribe European politicians and make luxury purchases. (BBC)