Pehlu Khan’s death made headlines when a suspected cow vigilante group was blamed for attacking him.
The cow is considered sacred by Hindus, and killing the animal is illegal in several states.
“We will only stop fighting if they kill us,” his son told BBC Hindi.
They family now plans to appeal the verdict in a higher court.
“As long as I am alive, we will keep fighting, we will not quit, even if we have to sell everything, we have to do it,” his son Irshad Khan told BBC Hindi’s Vineet Khare at his home in the northern state of Haryana.
A video of Khan being assaulted by a group of men in the north-western state of Rajasthan went viral on social media shortly after his death.
However, the court refused to accept the video as evidence since it was not verified.
Khan’s wife, Jaibuna Begum, said the family had been in mourning since the verdict came out on Wednesday.
‘How did Pehlu Khan die?’
Vineet Khare, BBC News, Haryana
It was 15 August, India’s Independence Day, when we arrived at Pehlu Khan’s house.
We could hear patriotic songs being played in a school nearby as his son, Irshad, explained how all his efforts of the last two and a half years to seek justice had come to nothing.
“What will we do in an India like this?”
Irshad said he was with his father on the day he was attacked and that he had spent all the time since his father’s death seeking justice – chasing lawyers, attending courts and organising protests.
During the conversation, he would st3ay quiet for long periods, constantly fiddling with his phone.
“If everyone has been acquitted, including those who were seen on the viral video in which my father was being beaten, then who killed my father?” he asked.
“While those who have been set free celebrate, we have lost everything,” she said.
After Khan’s death, his family sold off their cows to pay for their legal expenses. They had earlier told the BBC that they did not receive any compensation.
Many states have actively started enforcing bans on cow slaughter since the Hindu nationalist Bharatiya Janata Party (BJP) formed India’s federal government in 2014.
In addition to government bans, several vigilante groups who portray themselves as protectors of cows have also been active in several states.
The groups routinely check vehicles and often beat up cattle traders.
Prime Minister Narendra Modi has previously criticised the vigilantes, saying such people made him “angry”.
However, this has not stopped attacks against cattle traders.