You could hop over to the Coupon Connection and see what seems to be the going rate for miles. It's worth reading the "FAQ / Guidelines" at the top of the forum. IIRC, there's at least some valid argument that you may be violating AAdvantage program rules if you accept any compensation (including barter) for the award tickets--so you and your brother ought to have your stories straight about how it's a "gift" from you in case he is questioned during his travel on those tickets (unlikely in the case of a family member according to posts I've read on the CC, but I haven't read them all, so ...).

For myself, I looked into this for my sister's benefit (shorter domestic flights not international)--I'm not comfortable with the idea of possibly stepping foul of the rules, so I just bought the tickets outright rather than use miles.

You could always just ask him to make an offer and decide yourself whether it's worth the trade.

Anyway, if your brother can trade you something of equal value, then why can't he trade that "equal value" item for cash (i.e., on e-bay) and buy the tickets himself?

Depending on where/when he's going in South America he could enroll in AAdvantage and do a Gold Challenge to net the required q-points for status thru Feb '09. So he gets the trip he wants, FF miles of his own to use, and AA GLD status for up to 20.5 months. Even at discount economy pricing, he could choose a routing for about $500 - $1,000 that would net 5,000+ q-points; for about double he could go full fare and net 10,000+ q-points and complete a PLT Challenge--all on one trip. (Assuming your brother lives near you, I used SJC to BOG to figure that, so going further south would compound the benefits).

Just a thought--it might be worth considering if your brother does enough flying to make it worthwhile.

cheers!