Is it permissible to continue working in a shop that deals in usury, given one's disability, old age, and difficult circumstances?

If your work in the shop is permissible, and you have no involvement with the usury practiced by the owner of the shop, then there is no harm for you in continuing that work, and what you earn from it is permissible. This is supported by the Prophet’s (peace be upon him) and the Companions’ dealings with the Jews despite their practice of usury, which indicates the permissibility of dealing with a wicked person, a disbeliever, or an owner of mixed wealth, as long as the transaction itself is permissible.