There is only a free upgrade, no downgrade. If you bought it for the XB1 first, you have to repurchase for the 360.
http://www.bungie.net/en/Help/Article/12122
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So did I lost my money? I'm really pissed off, is that logical Bungie? Can't I use the same credit to buy it in my 360? Did I just get robbed? because I am not being able to use what I payed for.
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Technically Bungie didn't have to even offer a free upgrade, you know. You can try calling Microsoft to see if they'll refund it and let you buy it on the 360