What is the ruling on the work of a travel agent if he knows that the customer will travel to commit an abomination?

Selling travel tickets is originally permissible. The employee is not obligated to inquire about the traveler's purpose. However, if it is certain or highly probable that the purchaser will travel for a sinful purpose, then it is not permissible for the employee to sell the ticket, as that would be assisting in sin. Allah Almighty says: "And do not cooperate in sin and aggression." The Prophet prohibited selling grapes to those who would make wine from them, and every action that leads to sin is prohibited.