With the Proposals module, the Checkout As sidebar has a Proposals entry. It lists proposals with filters All / Pending / Accepted / Partially Paid / Fully Paid / Past Due / Rejected and a Search by name or client box. Open a proposal to see its details, then press Collect Payment: the proposal's items (meals, labor & equipment, custom charges…) are loaded into the cart and you continue to the normal payment page. There, a Pay Deposit Only: X% toggle appears if the proposal allows a deposit. After payment the order is linked to the proposal (the order page shows Proposal: #id name and a Previously Paid line for later instalments) and the proposal status becomes Partially Paid or Fully Paid.