Is it permissible to give charity from one's personal allowance while there is a portion of Zakat on wealth that has not been fully paid?

If the money taken from the father or brother was through a legitimate means, such as a gift, then you are permitted to dispose of it through charity and other means if you are an adult. If the giver stipulates that it should not be given as charity, then this condition is void, and you are permitted to give it as charity. However, if you are not an adult, your act of charity is invalid and rejected.

As for paying zakat (definition) in installments after it has become due, this is not permissible due to the delay it entails. The principle is that zakat must be paid immediately once it becomes due.