The Short Version
The general principle that trust accounts hold buyer deposits and vendor advance payments is well-supported by multiple regulatory sources. However, the claim presents a specific end-to-end workflow—holding both fund types until completion, then releasing to the vendor upon a buyer's solicitor's "order on the agent" with commission deducted—that is not substantiated by any authoritative regulatory or legal source. Trust account rules and release mechanisms vary significantly by jurisdiction, and vendor advances are often disbursed as incurred rather than held until sale completion.