I am not a businessman and have no experience in the area, but a couple of things come to mind.
The white-label requirement suggests that they want to put their own name on it which suggests they will become a competitor to your company, or they see a market for it that your company does not. Perhaps your company should charge per user and/or per copy shipped in addition to the basic rate.
Charging an annual rate may not work if they use it internally. They could tell you that they are going with another product and keep using yours without telling you. You could start with the expectation that they will replace it in 3 or 4 years and make the contract for so many years.
If they want new features or configuration assistance, your company already has the experts and can bill them heavily for the work. Your company might end up using them as a continuing source of revenue in this way. They might also recruit your developers.
The software has different values: what it is worth to produce, what it is worth to a company to have it, and what that company thinks it is worth. If they are smart, #3 is close to #2 and that is the number you have to bid towards. It is worth more to the other company to have working software immediately than to wait a year developing the software itself. To repeat old advice, start with a high bid that is more than you expect to get and allow the other side to negotiate downwards.
You will be expected to supply good documentation and to fix any glaring bugs in the product, so finishing the sale may incur a $10k-$20k cost.
Whatever you get for it is a revenue that you did not have before. As long as it does not hurt your business for the other company to have the software, that should be a good thing.
If this one company wants the software, there might be other companies that want the software.
Get advice from someone more knowledgeable before following any of mine.