Glad you asked...
If you want to write a letter and ask for money then you will have to write the purpose why you need the money. Be honest and tell them that you will give the money back.
Then again, this answer might help you...
How to write a letter to ask for money depends on who you're asking. If you're writing a bank or loan company, you need to explain the reason for the loan, whether it's personal or for a specific reason and then why they should loan you the money. Why you are a good loan risk. If it's a letter to a friend or family member, you need to explain why you need the money and then if you plan to pay it back and how you plan to pay it back. Sometimes, you also want to mention why they should give you the money.

