Must the fiancée, who dissolved the engagement of her own volition after receiving money from the suitor to purchase wedding necessities, return the exact money, or return what she purchased with the amount, knowing that the suitor will be harmed by the return of the purchased items?

If the money was a gift to your fiancée for her to buy her necessities, then it is her property, and you do not have the right to revoke it, for the Prophet, peace and blessings be upon him, said: "It is not permissible for a man to give a gift or bestow a boon and then revoke it, except for a father in what he gives his child."

However, if the money was part of the sadaq (dowry) for her to buy her trousseau necessities after the 'aqd (marriage contract), and she bought them before the 'aqd, then she must return the full amount to you.

And if the money was part of the sadaq for buying trousseau necessities, and you did not restrict her in its purchase, then you only have the right to revoke those necessities that she bought.

It does not matter whether the withdrawal from the engagement was from your side or hers, because engagement is not a binding contract.