In Cairo, Egypt in the early 1800s, a slave could be freed by her master verbally before two witnesses or by a written document of emancipation or by being given the certificate of sale from her previous owner. Emancipation might be conferred out of simple generosity or for the promise of future financial compensation.
A binding promise of emancipation upon the master’s death could be made verbally before two witnesses or via a written document. Once an owner promised emancipation upon death, he could no longer sell the slave. If, when the owner died, the value of the slave being emancipated by his death exceeded one-third of the total value of the master’s estate, the master’s heirs could require the slave to pay them the difference.
– source: Manners and Customs of the Modern Egyptians by Edward W. Lane, first published in 1836