You will be okay. The tourist card rules seem to have changed slightly in that now they do seem to be taking them in when you exit Mexico at the Guatemala border.This past March they took mine in also when I left Mexico at El Ceibo. Had to spend the time and 262 pesos again when I came back north at La Mesilla.
But the good thing is that they allow you to keep your TVIP so that you do not have to pay for that a second time when you come back on the return leg of the trip.

Also know that at some of the border entries from the US they allow you to pay for the tourist card right at the border facility Banjercito but at others and those from Guatemala they issue the card but you still have to go to a bank and pay for it and get your tourist card stamped with the bank's PAGADO stamp.Actually they now issue a new computer printout page as receipt, and don't really stamp your tourist card , just more paper to carry.....but don't loose it or the guys at the exit border will have to do a computer search to see if you did pay up..
Best is to hand in BOTH tourist card and TVIP when you leave Mexico when there is no likelihood that you might come back within the remaining valid time on these permits.
Even if you have neither you can still spend time in Mexico in the border zone.

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http://advrider.com/index.php?thread....207964/page-5 then scroll down to post #93