The 'Anger Management' actor had been speaking to a policeman in Hermosa Beach in California, when he discovered the officer's 10-year-old daughter, Jasmine, had been diagnosed with Childhood Rhabdomyosarcoma (CR).
Father-of-five Charlie told the officer, "No parent should have to watch their kid go through that", and the next day sent a cheque for $75,000 to the Hermosa Beach Police Association, which had started a fund in Jasmine's honour.
According to website TMZ.com, Charlie's stunt-double Eddie Braun also gave $25,000, and the the family is extremely grateful for the donation and hopes their actions will inspire others to join the fight against CR.
CR is a debilitating form of cancer that occurs in the muscle tissue and requires over a year of expensive medical treatments like chemotherapy and radiation therapy.
Last month, Charlie gave Lindsay Lohan $100,000 when he heard she was having financial difficulties, but claimed she had never thanked him.
He said: "She got shorted and I found out, so I said, 'Here.'
"I'm still waiting for a text to say, 'Thank you'. Anything you know?"