[Delphiza]

I agree with some of the advice, but will offer a different take to give you something to think about.

For this single large company enquiry, don't worry about monetising the app yet. Having a single license sold is not really monetisation.

Think about it as a consulting gig on making changes to an OS project. The fact that you are the maintainer is a knowledge and familiarity advantage, not a control one.

Work out how much time it will take to make the changes, double it, and send them a quote. If they buy, make the changes, push an update, and invoice.

Take your time to decide how to monetise the product. For now, just get a good fee for your time. See if you fall back into the project before focussing too much on the business side.