Without them knowing you created said software.
Hold your breath, first check how many clauses of your contract you have breached already.
Unless I'm very wrong, you used "insider" knowledge to design and create a software, and now you want to "sell" it to your employer.
I already informed my boss of the offer.
You might be in trouble already, I'll advise
- Check your contract
- Consult with a professional lawyer
Note: In most cases, any artifact (code, script, document, prototype etc.) developed using either (1) company resource (hardware or software) (2) Company-paid time (3) Company specific knowledge already belongs to the company. It is not even "yours", to begin with.
In some cases, even the work done in your own time also is owned by the company already. Unless you are really really sure that the product you're claiming to be yours actually belongs to you, I'd say, it's a very risky idea.
I did this with the encouragement of my boss who is chief of our department.
Unless that "encouragement" was in form of a written approval to use the knowledge and resource, I don't think it has much value.
He has a lot of say in what software the company will buy
This is also a red flag. Someone who has a decision making power, usually refrain from making suggestions/ dropping hints about a potential product.
What should I do?
In the given order
- Talk to a lawyer
- Immediately remove yourself from all sorts of association / involvement with the "product"
- Just accept the offer from the other company and forget you ever thought of this selling idea.