Just do what I did. Bring in the receipt for the Target iPod you purchased and explain to them that you'd like to keep the one you got at BB but the Target one was less expensive.
When I did this they happily price matched Target for the 10% off. Then, interestingly enough, when I returned the unopened iPod to Target they somehow refunded me the full purchase price so I wound up getting my 16GB Touch for about $340 including tax.
As per the BB comments, definitely if the unit is abused BB has every right to refuse the return. On the other hand you generally can return something you bought there within 30 days as long as it includes all of the packaging, etc. When I returned one I re-wrapped it in the plastic wraps and they didn't even check it out to verify that the serial numbers matched, etc (it was in perfect condition anyway and had the bad screen before the FW update came out).
I definitely don't condone abusing products and then returning them (that drives up costs for the rest of us) but Best Buy does have a pretty liberal return policy.