Based on a Naaleh.com shiur by Dayan Shlomo Cohen
According to Jewish law one cannot buy a davar shelo ba l’olam (something that doesn’t exist or is intangible) because there is no gemirat daat (final comittment). If there’s no gemirat daat, one cannot make a kinyan (acquisition).
You can’t sell something that doesn’t exist yet because the buyer doesn’t know what he will get.
For example, if a deal is made on next year’s crops, the buyer can renege on it. However, once the supplier provides the crops, the transaction is valid because then there is gemirat daat and the kinyan has been finalized. If you buy a field together with the crops of the coming year, the sale is valid because you are acquiring the land that will produce next year’s crop.
Similarly, the poskim discuss a case of a cow and its unborn calf. The seller sold the cow along with the calf. By the time it was born, prices had changed and the seller realized he had sold it too cheaply. If there was a kinyan while the calf was in his mother, he can’t ask for more money. The price originally agreed upon is binding. However, if there wasn’t a kinyan, since it was a davar shelo ba l’olam, the seller can ask for a higher price.
Selling items that are still in a supplier’s warehouse is a problem in halacha because the seller doesn’t own it yet. There is no gemirat daat. Therefore, the deal is invalid and both sides can demand a different price later on. If the seller personally obligated himself to supply the items, he must do so when they comes into his possession.
Halacha dictates that custom can create gemirat daat. The buyer trusts the seller, even though he can’t supply the item right away. Since deals involving future commodities are made all the time, there is gemirat daat and the kinyan is binding.
According to halacha, if you buy a piece of land, the property belongs to you including its airspace above until the heavens. For example, if you bought a courtyard and someone on the third floor above your property wants to build, you may stop him. However, Jewish law teaches that where one person stands to gain and the other one doesn’t lose, “Kofin al middot sedom,” beit din can force him to allow the person to build. Where there is even a small loss, this principle does not apply and the person who wants to build has to pay for the building rights.
Although the Shulchan Aruch rules that copyrights and trademarks are intangible, the majority of poskim today maintain that you can own a copyright. In certain cases, a person who steals a copyright or trademark is considered a thief. Since trademarks and copyrights are commonly bought and sold, any deal involving these things is valid.
When you know what the assets are but you don’t know how much there are, the transaction is valid. If it turns out that there was much less than the buyer anticipated, he can make a claim that he was overcharged. If it turns out to be much more, the seller can claim he undercharged.
The halacha may seem that one who receives a mistaken gift from an inheritance can keep it, because you can only acquire what you know is there. However, with regards to an inheritance, you don’t need to make a kinyan. The moment the father dies, there’s an automatic transfer of ownership to the sons. Therefore, if a son gives someone a gift from an inheritance, not knowing its true valuable, it must be returned.