Converter coupon was refunded for cash/credit! Not allowed is it?

I bought an Insignia brand converter box at local Best
Buy abt two weeks ago.

I used one of my $40 coupons and paid the difference
with credit card.

I took it back to get money back .. and fully expected
to ONLY get my out of pocket money back.... but instead
credited my Visa card with FULL amount of the box....
abt $60 when combined with my coupon and the out of
pocket cash.

What gives?!   I thought once you used the coupon there
was no way to get the value back if returning an item?
No?