Si vous tenez absolument a payer.....payez....:

Visitors leaving
Mexico must first stop at Immigration. Bus passengers get off the bus, without luggage, and present their passports with FMM to the immigration officer. The Officer will keep the FMM, and stamp & return the passport.
Whenever I have crossed, he has always asked (in Spanish) for money. Years ago, it was only 100 pesos or US$10, but now it’s 300 pesos.
Beware! This could be a scam. If the DNI fee has already been paid, there is nothing to pay at the border, and here’s where you might have to show the receipt from the airline or bank.
THERE IS NO DEPARTURE/EXIT FEE — this has been confirmed by the Embassy of
Mexico and the Mexican Honorary Consul in
Chetumal
PS:Je suppose que ceux ayant payés ont un reçu officiel.....????? Ce serait merveilleux de le scanner et le montrer ici