Yes. On the proposal, click Make Payment under Totals (or + Add Payment on the Payments tab). The same checkout window opens in admin mode with an extra Checkout As selector to pick which attached customer is paying. You can choose any enabled payment method — including cash/other offline methods to record a payment that was received outside Sprwt — apply a deposit option, tip, coupon or the customer's digital balance, and click Checkout. The payment is saved against the proposal, logged as Payment recorded in the Activity tab and updates the status exactly as a customer payment would. The button is hidden once the proposal is Fully Paid.