Is it permissible to sell a cow on condition of buying sheep, or to exchange sheep for ewes with an additional payment?

As for the first issue: This condition was included by some scholars under "two sales in one sale," which is a prohibited transaction.

As for the second issue: There is no harm in selling an animal for an animal with a surplus or with an additional cash payment, if it is hand-to-hand by agreement. If it is on credit, then this is permissible according to the preponderant view, based on the statement of Imam Malik in Al-Muwatta’: "The agreed-upon matter among us is that there is no harm in (exchanging) a camel for a camel of its like and an additional amount of dirhams, hand-to-hand." Also, Al-Qurtubi said: "In this is proof of the permissibility of selling an animal for an animal with a surplus, in cash, and there is no disagreement on this, and likewise for all other things, except for that which is forbidden to have a surplus in its cash value from usurious items." And Al-Shawkani said: "There is no disagreement among scholars regarding the permissibility of selling an animal for an animal with a surplus, if it is hand-to-hand. The disagreement is only concerning selling an animal for an animal on credit." Indeed, Ibn Umar bought a riding camel for four guaranteed camels on his account. And Ibn Al-Musayyab said: "There is no riba in animals: a camel for two camels, or a sheep for two sheep, on credit." And Ibn Sirin said: "There is no harm in (exchanging) a camel for two camels on credit."