Imam al-Shafi'i (may Allah have mercy on him) said: "Because by the saying of the Prophet (peace and blessings be upon him), 'Whoever makes a salaf (salam sale), let him make the salaf,' he (peace and blessings be upon him) meant: 'Let him pay (the price).' The term taslif (advance payment) does not apply to it until he pays him what he has to pay in advance before he parts from the one with whom he made the salaf contract."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / Q11109