Record an advance payment
Take money up front and keep it as credit until the customer buys.
Open the customer
Go to Get Paid → Customers and open the person paying in advance.
Tap Record Payment
Tap the blue Record Payment button near the top of their profile.
Enter the amount and method
Type how much they're paying and how (cash, transfer, etc.). If it's more than they owe, the payment clears any open bills oldest-first and keeps the rest as advance credit.
It's held as credit
The leftover sits on their Store Credit tab as advance credit. Spend it at checkout or against their next invoice whenever they're ready.