Every so often, the following joke (or variations thereof) winds up in my email.
Bob, a good friend of Bill, calls on Bill's pretty wife Sue one afternoon while Bill is away at work.
Bob tells Sue that he's always wanted to sleep with her, and offers her $50 to do so. She refuses, so he offers her $100. This continues until eventually he offers $500, and she relents. Bob pays the money and they do the dirty.
That evening, when Bill returns, Sue tells him "your friend Bob called today. That man has some strange notions". Bill replies "yes he does. By the way, did he give you the $500 he owes me? He said he'd do a turn around today to drop off the money."
The joke isn't very funny, but it has considerable longevity. Indeed, anyone who's read Chaucer will recognise that this is a simplification of the Shipman's Tale, and there's no reason to assume that the basic joke isn't a great deal older than Chaucer's version.
But it's not the age that interests me (after all, several such situational jokes can easily be traced back to the Greek comedies), but the rather significant change in emphasis between Chaucer and the modern version. In the modern version, Sue is very much the dupe. As a morality play, the moral is at best "don't compromise your principles for money, you'll lose". In Chaucer's version, though, Bob is the dupe. Sue spends the money, and Bob only learns that it's been paid to Sue when he confronts Bill about this, and is then stuck with the fact that, since he never told Sue about the loan, Sue has no reason to assume the money isn't a gift. Chaucer's moral is largely about the flow of information in a marriage.
Which leaves me with the question of when the shift in emphasis happened? Much as I'd like to blame the Victorians (who don't get blamed for enough of the ills of modern society, I feel), I suspect it's a much more recent reworking, but can't find any evidence either way.