She has to offer expiation for killing but she cannot free a slave or fast, so can she feed a number of poor persons?

The expiation for killing someone by accident is to free a slave. If that is not possible then it is fasting for two consecutive months. The one who cannot do that does not have to do anything, and feeding the poor is not an option in this case, according to the more sound opinion. 

Allaah, may He be exalted, says (interpretation of the meaning): 

“It is not for a believer to kill a believer except (that it be) by mistake; and whosoever kills a believer by mistake, (it is ordained that) he must set free a believing slave and a compensation (blood money, i.e. Diya) be given to the deceased’s family unless they remit it. … And whoso finds this (the penance of freeing a slave) beyond his means, he must fast for two consecutive months in order to seek repentance from Allaah. And Allaah is Ever All‑Knowing, All‑Wise”

[al-Nisa’ 4:92]

The scholars of the Standing Committee for Issuing Fatwas were asked: Do I have to fast after paying the diyah and how many days should I fast? Are the fasts to be observed consecutively or can they be broken up, or can I give food instead? 

They replied: You have to offer expiation for killing by accident, which is freeing a believing slave. If you cannot do that, then fast for two consecutive months. It is not valid to break up the fasts, and feeding the poor instead is not acceptable in the case of expiation for accidental killing, because there is no evidence for that with regard to expiation for killing in the Book of Allaah or in the Sunnah of His Messenger (peace and blessings of Allaah be upon him), and your Lord is never forgetful. End quote. Fataawa al-Lajnah al-Daa’imah (21/273). 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked: There is a man who has diabetes, and he has to offer expiation by fasting for two months – the expiation for accidental killing – but he cannot fast because of his sickness. What should he do? 

He replied: If he is unable to fast then he does not have to do anything, because the expiation for accidental killing can only be freeing a slave or fasting for two consecutive months, as it says in Soorat al-Nisa’ (interpretation of the meaning): “And whoso finds this (the penance of freeing a slave) beyond his means, he must fast for two consecutive months in order to seek repentance from Allaah. And Allaah is Ever All‑Knowing, All‑Wise” [al-Nisa’ 4:92]. Feeding the poor is not an option here. If he is able to fast them he must fast, otherwise it is waived in his case. 

End quote from Liqa’ al-Baab il-Maftooh 107, question no. 24. 

He was also asked: 

There is a person who has to offer expiation for killing, and he cannot free a slave or fast. Can he feed the poor instead, or is expiation waived in his case? 

He replied: No, it is waived in his case. The expiation for killing is of two types only: either freeing a slave or fasting, because Allaah, may He be exalted, did not mention a third option. If the third option had been obligatory then Allaah would have mentioned it, as He mentioned it in the case of expiation for zihaar (a jaahili form of divorce): freeing a slave, and if that is not possible then fasting for two consecutive months, and if that is not possible then feeding sixty poor persons. But feeding the poor is not mentioned with regard to expiation for killing. Based on that, we say that if a person is unable to free a slave or fast, then it is waived in his case. End quote from Liqaa’ al-Baab al-Maftooh 190, question no, 15. 

And Allaah knows best.