Please don't make comments like this, particularly in threads where OP is probably misunderstanding their legal situation and about to be extremely disappointed.

It is absolutely routine for employment contracts for software developers to transfer the corresponding IP to the employer. It's the main asset the employee is being hired to create!

It is common in fair and reasonable employment contracts for the standard for what is covered to be something like anything done on work time, using work resources, or related to work activities. The legalese varies from place to place, but if the employment contract includes wording like "in the course of your employment" then it probably means something along these lines, again depending on your local laws.

Some employers do try to include much more wide-ranging grabs, potentially any IP created by a salaried employee during the period of their employment. I normally recommend against signing any contract that includes these, not least because it says something about what type of employer you're dealing with. It's true that the situation is less clear in this case, because in some jurisdictions such terms might be considered overly broad and so not stand up to challenge, but you really need a local lawyer to advise you about this because the rules vary widely from place to place. In any case, since OP has told us that the project in question is directly related to what they do at work, this aspect probably isn't relevant to today's discussion.

OP, I'm sorry to be the bearer of bad news, but unless there are important details you've omitted or your employment contract is unusually liberal, you probably don't have much of a leg to stand on here. It's quite likely that your employer has actually owned all of the relevant IP from day one.

That would probably mean you wouldn't be entitled to any sort of compensation for it. (This isn't to say that your employer might not offer something to maintain good will and keep you on-side, but that's a different matter.)

More than that, it would potentially also mean you never had any legal right to open source it yourself, if the copyright was never yours so you never had any power to license it. That in turn could mean anyone redistributing it has been infringing your employer's copyright all along and the employer could even sue them. The employer could also take the whole project back closed source, or do anything else legal they want to do with it. (Again, this isn't to say they will actually do any of these things, but the employer being nicer than the worst case isn't a problem.)

On top of all of that, if the project is in any way in competition with what you do at work, you may have violated other conditions of your employment, which as with so much of this stuff may end up changing nothing or could have quite profound implications.

Short version: You do need a local lawyer who works in this field, right now. They will help you to understand your real situation, which no-one here can do properly. They may well advise you to simply hand everything over, and perhaps to seek some sort of written confirmation from the employer that they don't consider you to have violated any other aspects of your employment contract and the matter is then settled. If your employer is into open source and sees the value in the community you've built, you might get away with that. On the other hand, if they're sufficiently aware and litigious to go after your project in the first place, things could be much worse, and then you definitely want to have proper advice before you do or say anything else.