Technically, a buyer could pay closing costs in US dollars. The catch is that the notario must settle all the associated fees and the acquisition tax in Mexican pesos (MXN), which means your dollars would have to be converted. Notarios tend to apply a rather unfavorable exchange rate to protect themselves against any sudden swing in the market.

For that reason, paying everything in pesos almost always works out better for the buyer. On closing day, the escrow company converts your USD or CAD into the exact amount of pesos needed for the closing costs, using that day's exchange rate.