I can see the flexibility of wanting to join as an employee whilst your product was purchased by your employer rather than as a founder/director, but I question your employer's motives for proposing this.

It's still not clear if you were paid for the acquisition or it was a gift from you. If your salary is sort of counted as payment for the product, then you are working for free. Do you see the issue?

In short I would be pretty annoyed, because they appear to have let's say convinced you that this is a good deal when in fact you stand to lose quite a lot. If the product is so good they better be compensating you way beyond a normal salary.

If they are compensating you well then actually you might be on to a winner, in the sense that you already have a potential buyer for your next venture, but you need to get the agreement to build a Proof of Concept that you own. That way you give them the choice but retain ownership until you want to release it.