Night shift diff

Looking for advice on how to handle this situation. I am currently a FNP w 1 year experience in community health center and 5 years experience as CVICU RN. I was recently hired to work with a hospitalist group. The hospitalist program is new at the hospital and they currently are looking to hire at least 4 NPs/PAs but now only have 2 ( I am the second). I prefer to work nights and communicated this in my interview, however, they insisted that I would have to work days for a while until I am trained and they hire more staff. Upon taking the job, during my first week, I was discussing w the MDs that I prefer nights and will work them once we are staffed. The MDs were excited and the medical director immediately approved for me to start nights because there was such a need. I agreed and emailed the clinical manager that I would be working soley nights and inquired about the night shift diff. After 5 days, she emailed me back and stated simply "no night shift diff". I could kick myself b/c hindsight, I have learned I should have negotiated this scenario and had it written in my contract, however, it never dawned on me that they would not pay a shift diff and it was made clear that I would be working days for some time. I feel taken advantage of and cannot get comfortable w not being compensated for working nights. My question is how do I discuss this issue w my clinical manager w/o getting on her bad side, but also making it known that I should be compensated. I feel like she knows I prefer nights so she is not going to be willing to pay me. Key points are

1. there is a need for nights and no one wants to work them-I do

2. From my understanding, it is standard for nocturnists to be paid night shift diff and had the position been posted as nights, a shift diff would have been offered..that was my experience when interviewing at other hospitals, and I know of NPs in same hospital system being compensated

3. Help lure more experience NPs (they are having a problem w currently) that do not want to work nights b/c I would be doing that

4. Retention-they are not going to want to spend the time training me and then I ultimately leave to go to a facility where I can work nights and be compensated