How to formulate a reminder regarding letter of financial support?
I would write something along the lines of: 


  Hello Professor, 
  
  I am writing to follow up on the letter of financial support we had
  discussed a while back. I was wondering if you had a chance to write
  it, or if you needed more documentation from me. Please let me know if
  I need to do anything on my end. 
  
  Best,  Nikki