The weak peso is complicating transactions involving Mexican companies, driving up prices in local currency for acquisitions abroad or creating an unforeseen bargain for a foreign buyer, with at least one deal being revalued to adjust for the changes.

When Mexico City-based Coca-Cola Femsa agreed, with partner Coca-Cola Co., to acquire the AdeS soy-based beverage business from London-based Unilever for $575 million in June, the deal was valued at 10.6 billion pesos. Now the peso-denominated cost is about 9% higher. Like other Mexican public companies, Coke Femsa has at least some hedges for currency risk, but it has to book its holdings in pesos for accounting purposes.

Likewise, Grupo Lala said in May that it would buy certain U.S. assets from Laguna Dairy for $246 million. Currency weakness has added more than 10% to the peso-denominated price tag for the dairy company. The deal's dollar price remains unchanged, Alberto Arellano, Lala's chief financial officer, said in an email.