What is the expiation for a father borrowing money from a bank with interest to buy a new house, knowing that he was compelled to do so?

Expiation for the sin of usury and other sins is achieved through seeking forgiveness (Istighfar) and sincere repentance (Tawbah (definition) Nasuh), which involves abandoning the sin, regretting having committed it, and resolving not to repeat it. Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" As for benefiting from the house itself, there is no harm in it.