It’s pretty common for corporate stuff (legal or otherwise) to start with no payment changing hands, just a contract. Then an invoice lands either monthly or on completion afterwards.
That makes it easier for the work to actually start (otherwise you need to engage the finance dept up front and they’re often slow), and once the contract is signed and the work started that’s the sales process complete.
My colleagues having a chat about their favourite tv shows in the operations channel at 7am have entered the chat.