Is expiation obligatory for one who, in a moment of distress, said to his wife: "You are forbidden to me," and is it permissible to pay it in cash to the mosque instead of feeding sixty poor people?

A man's statement to his wife, "You are unlawful to me," is an implicit expression (kinayah) that requires intention. If he intended ẓihār (a form of divorce by injurious assimilation), then it is ẓihār. If he intended divorce, then it is a divorce. If he intended an oath, then he is liable for the expiation of an oath. The view that making one's wife unlawful constitutes ẓihār is held by some scholars. Therefore, if you paid the expiation of feeding sixty poor people after being unable to fast for two consecutive months, then what you did is correct, Allah willing. However, if you paid it while being able to emancipate a slave or fast for two consecutive months, then that does not suffice, because the expiation for ẓihār is obligatory in sequential order, as mentioned in the noble verse: "And those who declare ẓihār concerning their wives and then wish to go back on what they said—then [there must be] the freeing of a slave before they touch one another. That is what you are admonished thereby; and Allah is Knowing of what you do. And he who does not find [a slave to free]—then a fast of two consecutive months before they touch one another. And he who is unable to do so—then the feeding of sixty poor persons." Merely placing it in the mosque is not sufficient; rather, it must be delivered to the poor.