The builder wants money up front and I don't know my exposure
The payment is due before the work or goods are in your hands, and it is hard to see how it adds to everything else already committed.
What's happening?
A deposit or advance payment changes the sequence: money leaves before the goods arrive or the work is complete. The exposure depends on the amount, the date, the party holding it, what is received at that point and the terms that describe ownership or protection.
Looking at one request in isolation can hide the peak exposure across several orders. A dated register shows those payments together and creates a factual basis for asking what sits behind each one before the money moves.
The solution
List every advance payment, its timing and its supporting documents before agreeing the next one.
If you would rather it were done for you, Deposit and Advance Payment Review is the service that does it.