Is it permissible to work for a government institution that sells land for cash or in installments with a specific interest, and what is the legitimate proof for that, and does this apply to working in banks?

If the institution sells real estate on a known deferred payment basis at a price higher than the immediate cash price, this is an installment sale, which is permissible. However, if it deals with an interest-based system, by increasing the price on the buyer when he delays in payment, this is not permissible, and it is forbidden to work with them because it constitutes cooperation in sin.