Within the United States, it is legal to referral clients to other agents, as long as the agent belongs to a Brokerage. The referral will actually go to the brokerage. The referral fee should be agreed upon before you refer the clients over. This is done quite often. My company has a Relocation director, her job is to find reliable/reputable real estate brokerages in different cities, states, countries to refer clients in and out of area.
There are also many international relocation companies that work with different real estate brokerages in various locations. Our relocation director work with them also.
Don't know if that's legal in Mexico, I guess you will have to find that out.
If you need detailed information, I can have her contact you.