> They originally obtained the property by deceit so it's fraud.
They originally obtained the property by purchasing it. No fraud there. What is fraudulent is that they obtained the refund by promising to return the property, and then refused to actually return it after receiving the refund. That would be similar to accepting payment for a purchase and then failing to deliver the product.
Whether that counts as theft depends on whether you consider the title to the property to have changed hands upon completion of the refund--in which case the buyer is withholding property belonging to someone else--or whether the refund and the change in ownership of the property are two separate events connected by a mere promise. The more consistent models tend to treat all trades as belonging to the former category, meaning that backing out of the transaction halfway--accepting the refund but not returning the property--is both fraud (by intent) and theft (by action).