Deposits and money on account
Unallocated, on their account, until there is an invoice to put it against.
Take the money as an ordinary receipt and allocate none of it. It sits on the customer's account as credit you are holding. When the job is invoiced, open the payment and allocate it.
Where to see it
The customer's account panel shows what they owe and what is sitting unapplied, separately. On the debtors report the unapplied amount is netted into their current column, and the total says how much of the workshop's debtors figure is actually money already in the bank.
Unapplied credit is not the same as a credit note. Credit is money you are holding; a credit note is an invoice given back. The first is a payment, the second is a document.