Can the money designated for charity, which is placed in the post office, be considered a disposal of the post office's interest, knowing that the interest is added to the balance and not withdrawn?

The interest on a loan is usury (riba), which is forbidden by consensus, because every loan that brings a benefit to the lender is unlawful. One must repent to Allah and dispose of these interest gains by spending them on charitable causes with the intention of getting rid of unlawful wealth, not with the intention of spending in the way of Allah. Likewise, one must repent from depositing money in usurious post offices unless there is an extreme necessity to safeguard the money and no other means are available. In this case, the value of the interest must be estimated and disposed of by spending it on charitable causes without the intention of giving charity.