Is it permissible to deal with usurious banks for selling computers to customers, and what is the ruling on returning surplus funds from government department budgets in cash to purchase other unbudgeted necessities?

The first transaction is not permissible if you know that the customer will borrow with usury, due to the Almighty's saying: "And cooperate not in sin and aggression," and due to the Prophet's (peace be upon him) saying: "Allah has cursed the one who consumes Riba (usury), the one who feeds it, its two witnesses, and its scribe," and he said: "They are all equal."

As for the second transaction with government-affiliated departments, there is no harm in it if you return the remainder to the department, and the department must adhere to the state's conditions for disbursing what has been allocated to it.