How can one absolve oneself from the sin of using office supplies (such as pens, rulers, calculators, etc.) taken from previous workplaces, knowing that their approximate value has been given in charity? And is it obligatory to dispose of them?

If you have taken items without right, they must be returned to their owners if they are still present, or their value must be returned along with the value of any depreciation if they have depreciated. Your obligation is not absolved by giving them or their value in charity, except when you are unable to deliver them to their owners. If it is impossible to reach the rightful owners, it is permissible to give the value of the items in charity, and the sin of usurpation is lifted by paying it to the ruler or by giving it in charity on behalf of the owners while guaranteeing it to them.