Is it permissible, in Islamic law, to use personal usurious interest (ribawi) funds to repair a computer printer belonging to a public administration, and is it permissible for an employee to use this printer after its repair?

It is obligatory for a person to dispose of usurious interest by spending it on the general welfare of Muslims. Therefore, there is no harm in repairing the printer with it, even if other employees use it with the administration's permission, because the benefit here is incidental, not intentional. It must be noted that it is not permissible to maintain an usurious account except in cases of extreme necessity.