I just got into a contract with a organization, and I had them give me an initial deposit of 30% of the final price for services charged. They requested for a receipt. How would I go about creating that receipt? Do I just write a couple sentences stating this is a receipt for the deposit ($600) they made with my company name and date deposit was made, then signed by me on the bottom? I'll be sending them an invoice for the full amount when the project finishes but they need a receipt for the deposit which is understandable. Should invoice be count as a receipt but I'm going to send an invoice after project finishes?

Do you guys often send the invoice after project completes or at the beginning? I read around and people seem to send an invoice early in the process to give customer time to pay or an invoice for each step completed in the process...