Imam Malik (may Allah have mercy on him) says that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) was asked about freeing an obligatory neck (slave), and it was said to him: May it be bought on a condition? He said: No.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الظهار / 15273