Is it obligatory to donate the entire salary increase, or only the amount that was expected when the vow was made to donate it to the mosque?

The questioner must donate all the increase he received, because the wording of the oath he uttered encompasses the entire increase, and he did not have an intention to specify a particular amount. If he sees that he is in need of this increase, and that taking it is better than donating it, then he must expiate his oath and keep the increase.