Returns

 "I need a hen canary. I'd like that one please."
 - We don't have any hens at the moment. That one is a cock bird.
 "It looks like a hen to me."
 - It came in because it was a cock bird: the owner had too many spare males.
 "Hmm, I'm not so sure. I reckon its a hen."
 - We've also seen it singing
 "I think I'll take it anyway."
 - Okay, that's fine, but I am selling it to you as a male.
 "Fine, fine. I'm pretty sure it's a hen though."
 - Well, that's up to you. But we are selling it as male, you understand?
 "Yes."
 - And you are happy to buy this male canary, because you think it is a hen?
 "Yes."
 - Okay

Two days later...

 "Hi, I bought this canary from you a couple of days ago."
 - Yes...
 "Well, you told me it was a hen, but it has just started singing. Clearly it's a cock bird."