Is it permissible to work as an app developer for a real estate company, when one might be asked in the future to integrate a usurious mortgage service, knowing that his current work is not related to it?

If you are asked to create a link for a website, or to modify it, or to embed it in your applications, and most of the website's services are permissible, then there is no harm in it for you. However, if you are asked to make modifications that serve forbidden transactions, then this is not permissible, and you can stipulate to them that you will not participate in this type of modification.