Is it permissible to give a portion of the financial aid designated for a needy person to another poor person without informing the donor, especially given that this happens frequently?

Your ruling is the ruling of an agent. You must spend the money on what they appointed you for. If they appointed you to give it to a specific person or a specific category, you must adhere to that. If you do not find a rightful recipient, you must return it to them. It is not permissible to disobey them, as that is a betrayal of trust. Whoever disobeys their principal sins and is liable for the money. However, if the merchants gave you unrestricted discretion, it is permissible for you to give it as charity in all forms of almsgiving.