Dear Name (Friend Name),
I am deeply obliged to you for the financial help you have extended to me in time of dire need. I was hard pressed for it and I had tapped almost all sources. Your kind assistance has saved me much trouble. In what words should I thank you for this ? Of course, it is very difficult to have friends like you these days. This assistance will clear the obstruction that has held up the progress of my business. I will try my best to return your amount at my earliest possible.
Thanking you once again and with best wishes.