it is not a bill of exchange according to the UBS Dictionary of Banking, please see http://www.ubs.com/1/e/about/bterms/content_b.html
and search for "aval", you will find that bill gurantee ais your translation and bill of exchange is something else.

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Note added at 29 mins (2007-10-23 16:41:01 GMT)
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Karlotta, it may be that we are talking about two different things, so I am pasting 2 definitions below:

bill guarantee / Wechselbürgschaft / aval / avallo cambiario
Also referred to as aval. Undertaking of joint and several liability by the endorser (by signing the bill) to settle a bill of exchange in the event of the failure of the actual drawee of the bill to do so. See also guarantee.

bill of exchange / Wechsel / effet de change / cambiale
Payment order written by one person (the drawer) directing another person (the drawee) to pay a certain amount of money at a specified future date to a third party. It designates a named beneficiary but is transferable by endorsement. Discount credit when sold to a bank. See also promissory note, acceptance, draft.



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Note added at 1 hr (2007-10-23 17:20:31 GMT)
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Hello, did you see the 2 definitions above to clearify your doubt? bill guarantee> aval cambiario? Said that, only you should know what is right because you are the translator and you have the whole text. :-) Good luck.