And I'm not saying that I wouldn't too.
However, from the retailers perspective, I've paid a higher price for that lens and sold it at the profit margin that I deemed appropriate at the time. The customer willingly bought the lens and had no issue with said price at the time of purchase.
But faced with the choice of having the lens returned to me and potentially having to sell it as a second (depending on the condition of the lens and packaging) I'd be willing to listen to what the customer had to say.
But then again, I wouldn't be offering a 28-day returns policy on photographic equipment. Once you're past the 7-day cooling-off period and the lens has been 'used' I think it's fair to say that you're on your own.
If I was antc, I'd be checking the T&C's on the website and seeing where I stood. If I was within my rights to return the lens and receive a refund, I'd ask for them to credit me the difference. If I wasn't, I'd be asking for something back as a gesture of goodwill, assuming the price has dropped at the retailer in question.
And if it was a case of the latter, perhaps a credit note or voucher to the value of the price difference? That way the retailer ensures the money will be spent with them and the customer gets something they aren't entitled to, but that hopefully smooths over the cracks nicely.