Is it permissible to count the financial assistance given to a sick brother as part of Zakat?

The intention (niyyah) is a condition for zakat when it is given to the poor. If you did not intend it as zakat at the time of giving, then it is not permissible to count it as such. However, in the future, it is permissible to give to him with the intention of zakat, as long as he remains poor.