ومن باع عبدا على أن يعتقه المشتري أو يدبره أو يكاتبه أو باع أمة على أن يستولدها فالبيع فاسد
Translation: In historical slavery cases, a sale conditioned on emancipation, tadbir, or a manumission contract, or on having a child with an enslaved woman, is defective.
These are historical slavery-law examples, not present-day sale models.
The juristic issue is an extraneous condition attached to a sale: the seller requires a separate act that the transfer of goods itself does not demand.
In ordinary modern commerce, examine any added obligation to determine whether it is lawful, required by the contract, and accepted by the relevant doctrine.