Abu Amr al-Shaybani (may Allah have mercy on him) said: I found some runaway slaves. I came to Abdullah bin Mas'ud (may Allah be pleased with him) and told him, and he said: There is reward and gain. I said: The reward is there, but how is there gain? He said: Forty dirhams for each one.
This is the soundest of what has been narrated on this topic.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب اللقطة / 12125