I am confused about this.
Are you allowed to purchase development cards you can't use and put them in an empty barn?
I think the purchase rules require you to be able to use ("execute" in the English translation rules here on BGG) the development card to buy it. And purchasing is the only way you can get a card for the barn, right?
If you have an empty barn, the rules would not prohibit a development card from being purchased to place in said barn. Of course, if you're keeping it from another player, it would remain in said barn until the conclusion of the game.