I am the front desk of a four-star hotel, and a travel agency came to me and said that it could help me earn extra money.

That depends on what kind of extra income it is. If you just help the travel agency to contact individual customers and then give you kickbacks, this well-known problem of extra money is not big, but you must know the reputation of this travel agency and so on, so as not to be complained by guests because of the bad reasons of the travel agency. If it involves financial aspects, such as invoices, don't do it. You can't expect this money to make a fortune, it's against the hotel's professional ethics. If you have just been a hotel for a long time, I suggest you observe your colleagues around you first. See what they do, and you'll understand.