Has anybody heard this mix?
It's listed in the German Fan Club Collectors guide for around £60 and the seller is right - it's not on the Freddie boxset.
http://cgi.ebay.co.uk/Freddie-Mercury-RARE-MIX-Brazilian-12-Living-On-My-Own_W0QQitemZ330354455257QQcmdZViewItemQQptZUK_Records?hash=item4ceaa8f2d9&_trksid=p3286.c0.m14
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There is an alternative extended mix of this track, but it is not only from Brazil, it is also found on the US 12". Not in the box set as you mentioned.
As above - would only add it's not worth that much - half would be about right