There aren't really any sample letters for requests of this nature. Your employer would probably appreciate a formal letter - especially if they will have to file it as an official advance of funds - that lays out very clearly your request including an exact figure. If you have a plan for repayment, you should also make this clear so that the letter can serve as the contract for the transaction. The letter should be courteous and addressed directly to your employer by name. It should not contain any more information than necessary i.e. Do not go into detail about why you are requesting the loan because this is simply a business transaction.
