Rav Adda bar Ahavah answers: The case is where he was gored and made a tereifah (deathly ill, where he was going to eventually die from his wounds). The “inheritors” referred to here actually means to the slave himself.
Rava retorted that there are two reasons to refute this answer. One is that the braisa says the money is given to his inheritors (not himself). Additionally, the payment is kofer, and Rish Lakish says that kofer is only paid after an actual death (not when someone is made deathly ill)!
The Peri Chadash asks: Why couldn’t the Gemora use the following case? He was gored and rendered a tereifah. The ox owner is required to pay the penalty after he dies. Before he died, however, the master emancipated him, he married and begot children. Afterwards, he died, and the ox owner should now be obligated to pay to his heirs!?
He proves from this that it must be that a tereifah is not capable of having children.
The Chazon Ish asks that even if we will assume that a tereifah cannot have children, there is another possibility. He was gored and injured so badly that he was dangerously close to death (yet he was not ruled to be a tereifah). The ox owner is required to pay the penalty after he dies. Before he died, however, the master emancipated him, he married and begot children. Afterwards, he died due to the injury, and the ox owner should now be obligated to pay to his heirs!?
He answers that a slave has no lineage, and therefore any children born from him while he was a slave are not regarded as his children. After he is emancipated, and now his children are considered his children, that is only for all matters that are applicable after his death. However, with respect to the obligation of the kofer payment, that is a payment that is owed to the victim’s heirs. If, at the time he was gored, he did not have any inheritors, the owner will not be obligated to pay to the heirs that came about at a later date.