This is also needed for the "10 gems = 10,000" too, because I've had it happen TWICE now, where I click to order something from the catalog, not have enough money (due to resizing/redecorating) and have it say "You can buy for 10 Gems" and I accidentally hit yes, because I'm so used to just 'confirming' that it's okay that I don't have enough money.
It needs to be NOT automatic, when people, like me, SAVE up all their mastery Gems to afford decorations only to have them vanish for an extra slot or only 10,000 coins.