Bonus Eligibility at the end of a temporary assignment.

I joined a disability law firm in Cherry Hill, NJ on 8/29/11. I joined the firm as the TEMPORARY HR Manager, while the regular HR Manager would be out on FMLA through the middle of January 2012. Part of the terms of employment was to sign an offer letter, (clearly stated that it was not considered an employment contract). The offer letter stated that my employ with the firm would end on or around January 20, 2012 at which time I would receive a 1 time lump sum bonus in the amount of $5,000. To receive the $5,000 I would need to represent the firm as the HR Manager through the duration of the regular HR Manager's FMLA and that I would need to complete the assignment in its entirety to include spending at least a week with the regular HR Manager upon her return to ensure a smooth transition back. In the middle of the regular HR Managers FMLA, she requested an extension of 2 more weeks of leave to bond with her newborn. So instead of returning on 1/16/12 as originally scheduled, she would be returning on 1/30/12. The firm never drafted a revision of the original offer letter that I signed on 8/30/11 to require an extension of my assignment beyond 1/20/12 as originally drafted. Towards the end of my assignment the regular HR Manager then requested an additional 2 days of leave beyond her revised return date of 1/30/12. This would extend her leave through 1/31/12 for her return on 2/1/12. Again, there were no revisions to the original offer letter requiring me to extend my assignment. On January 20, due to on-going health issues and working in what was a very stressful environment, I informed the firm that I could not continue for the remaining 2 weeks, but would come in for 2 days at the beginning of the coming week to wrap things up for them. I also offered to come in for a day the week the regular HR Manager would be returning to work with her for the day and assist with her transition back. The Operations Director, whom I reported to, agreed to that. I did come in and work on the Monday and Tuesday and in communication with the regular HR Manager told her that I would be coming in on 2/2/12 to work with her. Subsequently, she has told me not to come in since it would be her 2nd day back she would not know what I could immediately assist her with.I inquired with her and the Operations Director regarding my bonus eligibility and have been informed by the regular HR Manager that the Operations Director is not prepared to make a decision and has to give it some thought.I have not received a response to my inquiry via email from the Operations Director.I am concerned as to whether or not the firm can withdraw the bonus agreement as in their mind I did not stay with the firm through the entire duration of the assignment, but to reiterate there were no signed revisions to the original offer letter which would have obligated me to stay beyond the original 1/20/12 as stated in the letter that I did sign. And again... I did work beyond the 1/20/12 date because I did go in to work on 1/23/12 and 1/24/12 to wrap things up for the firm.