Job Promised Me $10k Bonus And Is Now Walking Back On It.

Back in June of this year I received a job offer for another company for $90k base. I took this to my manager and asked if they’d be willing to match it. After some back and forth they offered me $70k with a $10k bonus at the end of this year. Now mind you this is not supposed to be an annual bonus, just a one time lump sum that would then roll into next years salary.

After considering it for a while I decided to take their offer. Now that bonus was supposed to come in today and after not receiving it I sent them an email. They responded back saying that they would be prorating my bonus. So they are only going to be paying me $5833 instead of the $10k they promised. The exact wording on the agreement I signed with my manager was, “a structure of $70,000 base salary with a $10,000 bonus at the end of this calendar year to match the outside offer.”

Is this even legal? What are my options here? I still like my job so hopefully options that would allow me to stay here would be nice.