Can a portion of Zakat funds originally allocated for the medication of a sick girl—who no longer needs the money after receiving treatment at the state's expense—be redirected to other eligible recipients?

It is permissible to give zakat (definition) to a poor girl who is unable to afford medical treatment. However, the principle of zakat is that it must be given in kind to the deserving recipient, who then takes ownership of it. It is not sufficient to disburse it if the recipient's state of poverty ceases at the time of giving, even if you had intended to give zakat to them previously, because the poor person's ownership of zakat occurs upon receipt, not by intention.