There is an easier way to handle deposits, @Diana_Ceramics, built right into the program. See this Guide:
https://forum.manager.io/t/customer-deposits-and-advances/7093
As for how to request the deposit, the designed method is via a sales quote. Unlike the approach @Brucanna suggested, which creates an account receivable for work the customer has possibly not yet agreed to, sales quotes have no impact on your balance sheet. You can also rename the sales quote to anything you like, such as Proposal, Pro Forma Invoice, or Deposit Request. See this Guide: