It is not permissible to sell a specific fish in the water that has not been caught

Question: If a person specifies a fisherman to catch fish in the water, on the condition that it be of three types; for example, twenty kilograms of *kan'ad* fish, specifying a certain weight and a certain type, is this permissible?
Answer: If it is in his debt for a known amount, there is no harm in it until a known term, he catches and gives what is in it, there is no harm. As for a specific fish in the water that has not been caught and is not in his debt, it is not valid; because it is gharar (excessive uncertainty in a contract).