The Literal Commandment
Then you shall gather all its booty into the middle of its open square and burn the city and all its booty with fire as a whole burnt offering to the LORD your God; and it shall be a ruin forever. It shall never be rebuilt. Nothing from that which is put under the ban shall cling to your hand, in order that the LORD may turn from His burning anger and show mercy to you, and have compassion on you and make you increase, just as He has sworn to your fathers, if you will listen to the voice of the LORD your God, keeping all His commandments which I am commanding you today, and doing what is right in the sight of the LORD your God. (Deuteronomy 13:16-18)
The literal commandment is that nothing that is destined to be destroyed with the city that has turned to idolatry should "cling to your hand": we should not hold onto anything (even things of great value) that should be destroyed under these circumstances.