The fault lies with Square Enix for not implementing a way to sell houses. They are still learning as a MMO company, and didn't realize that relinquish fees will be the new thing when they said they didn't want profiteering. Which leads to situations like this.

You did what you agreed to -- what is the norm for situations like these -- and you are in the right.

In addition, one should always make sure they're ready to pick up a plot when agreeing to buy a relinquish. The only thing you could've done better is ask if he was a second lieutenant. But many wiki guides say that all you need to be is second lieutenant with any Grand Company. I honestly wouldn't have known this, myself.

As bad as you must feel, this is between two other parties now.