Is praying for forgiveness at dawn and during the well-known times of answered prayers and Hajj sufficient to atone for taking money from someone without their knowledge, given that returning the money might lead to enmity?

The Messenger of Allah (peace and blessings be upon him) said: "The hand is responsible for what it has taken until it returns it," and this means that the hand guarantees what it has taken until it returns it to its owner. Your obligation is not discharged except by returning the money to its owner. Your supplication and seeking forgiveness atone for the sin of theft, but the rights of people must be returned to their owners or you must seek their exoneration.