Zie onder 'transactie' in het Financieel-Economisch Lexicon: "zakelijke transactie - arm's length transaction". Uit mijn eigen ervaringen met het vertalen van NL naar EN, weet ik dat je 'zakelijk' vaak heel goed met '(at) arm's length' kunt vertalen. 'Onderhands' lijkt mij hier niet goed, want 'arm's length' betekent dat de partijen elkaar geen voordeel gunnen en dat de transactie op basis van gangbare marktprijzen plaatsvindt.

-> "at arm's length not closely connected.
to deal with someone at arm's length to deal as if there were no connection between the parties (as when a company buys a service from one of its own subsidiaries); the directors were required to deal with the receiver at arm's length."
Dictionary of Law, Peter Collin Publishing

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Note added at 20 mins (2004-06-09 09:32:09 GMT)
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-> \"arm\'s length transaction - A transaction between two related or affiliated parties that is conducted as if they were unrelated, so that there is no question of a conflict of interest. Or sometimes, a transaction between two otherwise unrelated or affiliated parties.\"
http://www.investorwords.com/254/arms_length_transaction.htm...