A transaction which is not considered in the Shari‘ah, which is regarded as useless, and regarding which one will say that it is as if the person did not even purchase the item and the seller did not even sell it, is referred to as batil or invalid. The rule with regard to such a transaction is that the person who purchased the item does not become its owner and that it is still considered to be under the ownership of the seller. Therefore, it is not permissible for the buyer to consume it or to give it to anyone. It is not permissible for him to utilise it in any way.
As for the transaction which has taken place but some sort of shortcoming is found in it, is referred to as fasid or improper. The rule with regard to such a transaction is that as long as the item does not come into the possession of the buyer, ownership of that item will not be transferred to him. Once he takes possession of it, it will come under his ownership but it is not halal and tayyib. Therefore, it is not permissible for him to consume it or utilise it in any way. Instead, it is wajib to cancel or annul this transaction. If the person wishes to keep it, he must recommence the transaction and then purchase it. If the person did not cancel this transaction and instead, sold the item to someone else, he will be committing a sin. However, it will be permissible for the second buyer to consume it or utilise it in any way and this second transaction will be valid. If the person sold it at a profit, it will be wajib on him to give the profits in charity. It is not permissible for him to use it for his personal purposes.
The custom of selling the fish that is in the lakes or pools of landlords is a batil transaction. All the fish that are in these lakes and pools do not belong to anyone as long as they are not caught and fished out. The person who catches the fish becomes its owner. Once you have understood this aspect, now try and understand the following: if the landlords do not even own those fish, how can it be permissible for them to sell them? Obviously, if the landlord catches the fish himself and then sells it, this will be permissible. If he asks someone else to catch the fish, that person will become its owner. The landlord has no right over the fish which has been caught by that person. In the same way, it is also not permissible for him to prevent people from fishing there.