We can't know. The question you need to ask yourself is: "Is this app going to make me more than $40,000?".
This is unfortunately a question which is impossible for us to answer. You say the app makes you $650 a month. If it would keep doing that, it would have reached that goal in 5 years. But will it?
You said it's growing, but will it keep growing? We don't know. Apps are a highly volatile market. The hot app of today might be obsolete tomorrow. Either because the app is riding on some short-lived lifestyle trend, because it will soon reach market saturation (anyone who needs the app already bought it) or because someone is already working on an app which does the same thing but better and has a larger marketing budget. You have to estimate if your app will stay relevant for years or not.
Or maybe your app is actually vastly underrated and waiting for its huge breakthrough? Statistically speaking that's very unlikely. But again, we don't know your app so we have no idea what its true market potential might be.
Also, will the app stay relevant without further work? How much of your own time do you plan to invest into supporting that app and how do you value an hour of your work? You need to subtract that from the income it generates.
But should you decide to sell the app, a few words of advise: Read the contract carefully! I have heard about people who got tricked into selling away their IP rights under really predatory conditions. For example a story about a publisher who tried to trick a developer into signing a contract where the developer was obligated to change and update the app for several years in whatever way the publisher demands without getting paid for it! Congratulations, you literally just sold yourself into slavery!
If you sell your app, make sure it's either a deal where you have no further obligations or one where you enter a business agreement where more work equals more money for you and where you have a reasonable exit clause in case you want to focus on something completely different.
When you are not 100% sure what you are actually signing up for or would like to propose a different arrangement, then it can be very helpful to seek advise of a professional attorney. When you are a freelanceer, then a good lawyer is part of the cost of doing business.