need advise on business proposal/contract

I run a small wholesale business and have come across a client with one time order of $500k above sale revenue. I sure don't have this kind of cash in my hand to first get product here from my vendor overseas and resale to this client. I was contemplating to get my client hooked up with the vendor so they can arrange ways to handle the finance part. So I need to sign some legal papers to safeguard my interests in the whole process. If any of you could shed some lights on this kind of business transactions, please do give your inputs here, anything would be greatly appreciated. Thanks in advance!!