The homogeneity of the goods requires that if both goods are to be sold in the Mexican market then they must sell at the same price in equilibrium. As these changes take place in the Mexican market, other changes occur in the US market. When US exporters of wheat begin to sell less in Mexico, that excess supply is shifted back to the US market. Thus at the end of each day, wheat supplies remain unsold. An inventory begins to pile up.
Producers realize that the only way to unload the excess wheat is to cut the price.
Thus the price falls in the US market. At lower prices though, US producers are willing to supply less, thus production is cut back as well. US producers now receive the same lower price for wheat whether they sell in the US or Mexico. Thus US exporters receive the US price for the wheat sold in Mexico. The higher price in Mexico raises domestic supply and reduces domestic demand, thus reducing their demand for imports.