There seems to be the first occurence on these pages of the perpetual confusion of ظ & ض . The Ms has موعضتكم instead of موعظتكمThe phenomenon is attested in the papyri since 709/10 CE.
The twin Psalter Brit.Libr.Harl.5786 Catalogus 8 ca 1153 also has استيقض in Ps 3:5.
Some relatively recent works on the phenomenon are :
Bosworth C.E. A Mamlūk text on the orthographic distinction of ḍād and ẓā’ // Orientalia Hispanica sive studia F.M. Pareja octogenario dicata. Edenda curavit J.M.Barral. Volumen I. Arabica - Islamica. Pars prior. Lugduni Batavorum: E.J.Brill, . P.135‒149.
Corriente F. D-L Doublets in Classical Arabic as Evidence of the Process of Delateralisation of ḍād and Development of its Standard Reflex // Journal of Semitic Studies. 23.1. 1978. P.50‒55.
Jonathan A.C.Brown. New Data on the Delateralization of Ḍād and its merger with Ẓā’ in Classical Arabic: Contributions from Old South Arabic and the Earliest Islamic Texts on Ḍ / Ẓ Minimal Pairs // Journal of Semitic Studies. 52.2. 2007. P.335‒368.