When is a merchant's continued receipt of goods and his employees' signatures thereon considered a tacit admission of the agreed price's suitability?

A merchant's continued receipt of goods and his employees' signatures thereon, particularly with recurring transactions and numerous invoices, is deemed a tacit admission of the price's suitability. He shall not subsequently be permitted to dispute the price on the grounds of his employees' collusion or their violation of his internal procedures, as the decisive factor lies in commercial custom and the apparent circumstances.