Bearing in mind Pasuk 24 - "Ayin Tachas Ayin", which teaches us the obligation to pay for injuries that one causes, why does the Torah see fit to insert this Pasuk?
Rashi and Targum Yonasan: To teach us that, besides paying for the actual injury, one is also Chayav to pay for work-loss and medical fees 1 .
"Shivto Yiten v'Rapo Yerapei". He is also Chayav to pay for the pain and the embarrassment (Targum Yonasan), which Rashi omits, because we learn them from another source (Sifsei Chachamim).
What is an "Egrof"?
Ramban and Targum Yonasan: a fist.
Ramban (citing Ibn Ezra, R'dak and Targum Onkelos: A clod of earth (which is also relatively soft).
Why did the Torah not refer to the implement with which Reuven killed Shimon in Pasuk 12?
Oznayim la'Torah: Because it is speaking about where Reuven planned to kill Shimon, in which case he certainly came with a weapon that kills, whereas here it discussing a case where Reuven and Shimon began quarreling and where Reuven did not plan to kill Shimon.
What is the significance of "be'Even O be'Egrof"? Why does the Torah see fit to mention them both?
Ramban #1: The Torah is coming to teach us that although the former is hard and prone to kill whereas the latter (a fist) is soft and does not usually kill, both require the assessment mentioned in the next Pasuk. 1
Mechilta: The dual comparison teaches that, just as 'Even' is capable of killing, 4 so too, must 'Egrof be capable of killing; and that just as a fist is availale, so too, must the Even be available, to preclude where it got lost. 5
And we neither assume outright that the former killed the victim, nor that the latter did not. If the victim subsequently dies, the accused is sentenced to death, and if he survives, then he has to pay (Ramban). And the Torah deliberately avoids mentioning a sword, which is always capable of killing, and does not need any assessment (Ramban citing Sanhedrin, 76b).
To preclude where the stone is mixed up with other stones and cannot be identified (Ibid).
Thereby obligating the stroke of the fist to be assessed - as to whether it was capable of killing the victim (Ramban).
See Torah Temimah, note 134, who refers to the Pasuk in Masei, 35:17 "Im be'Even Yad asher Yamus bah Hikahu".
The Gemara in Bava Kama makes virtually the same D'rashah using a different Lashon. See Torah Temimah, note 136.
What are the implications of "ve'Lo Yamus"?
Mechilta: It implies that if he struck hum a blow that could have killed him and he did not die but became bedridden, he is Patur from Sheves and Ripuy (until he is fit to walk in the street). 1
See Torah Temimah, note 137.
What are the connotations of "ve'Nafal le'Mishkav"?
Rashi: It means that the victim has been incapacitated and is unable to continue working in his regular occupation.