Is it valid to stipulate a general, unspecified slave as consideration in a writing contract?

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The stipulation of a general, unspecified slave is invalid, according to Abu Bakr and the view of Al-Shafi'i. The evidence for invalidity rests on the principle that what is invalid as consideration in sale or lease contracts, such as an unspecified garment, is invalid in a writing contract. Furthermore, a required animal in blood money (Diyah) is specified by genus and age, unlike the absolute specification here. Since an absolute animal cannot be the subject of a writing contract without specification, this also applies to the slave.