What is the ruling on selling a non-pregnant mare, with the condition that the first two or three offspring from this mare belong to the buyer, and does Imam Ahmad's exception apply to this case?

The four madhhabs (schools of thought) prohibit the exclusion of an existing fetus from a sale. Imam Ahmad, in one narration attributed to him, dissented and permitted its exclusion.

As for the exclusion of a future pregnancy, it has not been reported from any of the scholars, and it cannot be analogized to the exclusion of an existing fetus, as it involves a greater degree of uncertainty (gharar) and ignorance (jahalah).

The Prophet, peace and blessings be upon him, forbade the sale of "ath-thunaya" unless it is known.

The reason for the impermissibility of excluding the fetus is its unknown nature. The unknown nature of what is excluded makes the remaining part of the sale also unknown, and this prevents the validity of the contract.