My go-to volume on the [Cuerdale Hoard](http://www.britishmuseum.org/pdf/Cuerdale_online_2%20(1).pdf) lacks further useful information. The name Muḥammad ibn Aḥmad is fairly common, both then and now. Al-Banjhīr actually refers to Panjshir, which was the location of a mint near Kabul in modern Afghanistan. So you're looking for a ruler or governor named Muḥammad ibn Aḥmad from that part of the world, which in modern scholarship often gets lumped with Iran, or one of its medieval counterparts, most frequently Khurāsān.

It's probably worth looking up the Encyclopaedia of Islam entry on 'Pandjhir'. You'll probably have to visit the print edition. The Cambridge History of Iran also has a few paragraphs of certain relevance in vol. 4, pp. 372-3. The article refers to a dirham minted at Balkh in 909-10 by a certain Muḥammad b. Aḥmad b. Yaḥya, who might be your target. Alternatively, the British Museum has an entry for a Muḥammad b. Aḥmad who could also be your culprit. He's close in time and associated particularly with Panjshir. Up-to-date research can also be found in Robert Haug's recent dissertation, which is available online.