If that was a normal purchase, then that’s clearly theft.
If it was art leasing, there’s probably a long contract with details about a situation like this. No matter what the contract says, the local law might still disagree with that, so it can get complicated. The art company might be violating their own contract, although it is unlikely. The company might be within the rights outlined in the contract, but they might still be breaking the law. You need a lawyer to figure it out.