Even if there was agreement from the beginning that this is the company's IP, and you have to transfer control over it, remember you are under no obligation to continue to do what you are doing. If you were to stop now, no one else is going to support and maintain the code-case.
You have two other options:
1. You can stop immediately. Just say you are no longer maintaining and supporting it for free. You can work on it on company time. If anyone wants to pay you for your personal time to work on fixes and bug-fixes they wan, they can, at which point you can do that work, and release it as either part of the open-source project you have, or as a private fork for that customer.
2. You can quit your job. Go work somewhere else who is willing to work on your own open-source projects on your own time and maintain IP. Then, abandon the project since it no longer belongs to you. Your current company is the copyright owner. Start a fork, which will now be yours, and you're the copyright owner of any new code, including changes you made to the old code. If the original license is permissive, you are under no obligate to contribute back.
Having said all of that, this is not what I would personally do. I would be happy that the company I work on wants my work so bad that they're asking for the transfer of ownership back to them. I would tell them that shouldn't be a problem but you'd like something for it, such as being able to work on it on company time, etc. Otherwise it sounds like they just want to own this thing you created and have you continue to work on it for free on your own time indefinitely, which I don't believe is what they're asking for -- you weren't clear about that.
You have two other options: 1. You can stop immediately. Just say you are no longer maintaining and supporting it for free. You can work on it on company time. If anyone wants to pay you for your personal time to work on fixes and bug-fixes they wan, they can, at which point you can do that work, and release it as either part of the open-source project you have, or as a private fork for that customer.
2. You can quit your job. Go work somewhere else who is willing to work on your own open-source projects on your own time and maintain IP. Then, abandon the project since it no longer belongs to you. Your current company is the copyright owner. Start a fork, which will now be yours, and you're the copyright owner of any new code, including changes you made to the old code. If the original license is permissive, you are under no obligate to contribute back.
Having said all of that, this is not what I would personally do. I would be happy that the company I work on wants my work so bad that they're asking for the transfer of ownership back to them. I would tell them that shouldn't be a problem but you'd like something for it, such as being able to work on it on company time, etc. Otherwise it sounds like they just want to own this thing you created and have you continue to work on it for free on your own time indefinitely, which I don't believe is what they're asking for -- you weren't clear about that.