What is the ruling on taking a substance from a furniture store and placing it inside the furniture to protect it from damage, without the knowledge of the buyer or the owner of the goods?

Your act of taking the item without the seller's or buyer's knowledge is forbidden (haram) and a betrayal of trust. You must repent to Allah and return what you took to the buyer, because it is attached to the sold item and the buyer owns it through purchase. You must return an equivalent item, and if no equivalent exists, then its value. It is not necessary to inform the buyer; rather, what is required is to return the right to him by any means, while concealing your action. If you are unable to reach the buyer, then give the value in charity on his behalf.

If you stole from a person whom you do not know or whose whereabouts you do not know, then you may give what you stole in charity with the intention that it is for its owner.