To set up prepayment percentages for items, customers, and vendors
For an item, you can set up a default prepayment percentage for all customers, a specific customer, or a customer price group.

2. Select an item, and then choose the Prepayment Percentages action.
3. On the Sales Prepayment Percentages page, fill in the fields as necessary. Hover over a field to read a short description.
Reference:
https://docs.microsoft.com/en-gb/dynamics365/business-central/finance-set-up-prepayments