game #144443341
Played Haggler and bought Destrier, but cannot gain another destrier (which was cheaper than the first).
Later in the game, I bought Destrier for $5, but was not able to gain port ($4) because it was now $4.
That's a strange but correct interaction.
Haggler compares the current cost of the card you bought (and from which Haggler triggered) with the current cost of the card you want to gain. So in your examples, Haggler uses the current (that is reduced compared to what you paid) cost of Destrier.
For the first case, that means you can never gain a Destrier with Haggler from buiying a Destrier, and in the second case, the Destrier was now costing $4 and so Port is not available to gain.