Why does the man at the orphanage not want to admit he knew Sohrab?

Basically because he was instrumental in in selling Sohrab. he man did not seem to want to talk about Sohrab. When Amir insisted, he revealed that a Talib official had taken Sohrab a month earlier. This official came every few months and paid to take a child with him; the man had no choice but to consent, or he knew he and all his children would be shot. This news so enraged Farid that he tackled the man and tried to strangle him to death until Amir intervened. The man told Amir that he could find the Talib official at Ghazi stadium, where the national team played soccer.