If a fiancé says to his fiancée, to whom he is already married by contract, "Go, and I ask God to grant you the husband you wish for," does this count as a فسخ (dissolution) of the legal marriage contract, and is the girl still considered to be under his marital responsibility?

This phrase is not explicit in divorce; rather, it is an allusion (kinayah) by which divorce does not occur unless the husband intends it. This is based on the statement of Al-Shafi'i: "And if he said to her: 'Go, and marry,' or 'Marry whomever you wish,' it would not be a divorce unless he says: 'I intended divorce by it.'" The determination of whether divorce takes effect based on it depends on the husband's intention. If the spouses differ regarding the intention, then his statement is to be accepted because he knows his intention best. And unless the husband admits to the intention of divorce, the wife remains under his marital bond.