Sayyiduna Imran bin Husayn (may Allah be pleased with him) said that a slave boy belonging to poor people cut off the ear of a slave boy belonging to rich people. His people came to the Prophet (peace and blessings be upon him) and said: We are poor people. So he (peace and blessings be upon him) did not impose anything on them.
The Shaykh (may Allah have mercy on him) said: If the slave boy mentioned here means a slave (owned), then the consensus of the people of knowledge that a slave's crime is upon his own neck indicates, and Allah knows best, that this crime of his was by mistake. He (peace and blessings be upon him) did not impose anything on him, and paid it from his own side as a voluntary gift.
Abu Sulayman al-Khattabi (may Allah have mercy on him) said that the culprit was free and this mistake happened from him, and his 'aqilah (paternal kinsmen) were poor, so he (peace and blessings be upon him) did not impose anything on them; either because of their poverty, or because they do not bear the diyah for a crime committed against a slave, if the victim was a slave. And Allah knows best.
The Shaykh (may Allah have mercy on him) said: It may also be that the culprit was a free, under-age boy and his crime was deliberate, so its diyah would not fall on the 'aqilah, and he himself was poor, so it was not taken from him (at that time).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16366