How do I end a letter?

I'm writing a cover letter to a business partner for my boss. How do I end it? Yours faithfully? Sincerely? And how do I begin it? Dear X?

The problem is that I'm not sure how close my boss is to the business partner. I have gotten the impression they have never met but have been working with each other for the past six months if not longer.

The business partner lives in Franche but the lingua franca of the correspondence is English. Should I take his nationality into accord by being... more formal? Less formal? Any thoughts?


ETA: Thanks! I think one of these will work!