Is it valid when giving out zakat on wealth to say: "I give this money as zakat on my wealth, and whatever exceeds that is for the soul of everyone from whom I wrongfully took money, and whatever exceeds that is charity for the poor," or is the intention in the heart sufficient?

Intention (niyyah (definition)) is a condition for fulfilling zakat (definition), and its place is the heart. Uttering it verbally is not necessary, according to the more preponderant opinion. If the zakat payer gives more than the obligatory amount, he should have the intention of giving the excess as charity. Furthermore, what was taken wrongfully from others' property must be returned to them or their heirs. One's responsibility is not absolved by giving it as charity on their behalf unless one despairs of finding them or their heirs.