All perfect praise be to Allah, The Lord of The Worlds; and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Muslim jurists have stipulated that in order for the sale contract to be valid,the price should be clearly specified, as stated in the book "Moghni al-Mohtajj", "The compensation and the price have the same conditions which are: it has a value, it is known, and it can be delivered."
The prophet (PBUH) forbade all sales that are based on the unknown. Abu Hurairah narrated: "The Prophet (PBUH) forbade the type of sale which involves risk (or uncertainty) and a transaction determined by throwing stones."{Sahih Muslim}.
Therefore, the sale contract mentioned in the above question is invalid because part of the price-seven sheep: their weight, type etc-isn`t specified-, and this could lead to dispute between the two parties.
As a result, the sale mentioned in the above question is invalid and must be corrected by giving back the piece of land to its owner, and the money and the sheep to their owner as well. If the two parties wish to conclude a new sale contract, then the price should be specified from the very beginning. And Allah knows best.