City names in Iran and Central Asia, such as "Birjand" in Iran and "Khujand" in Tajikistan end with "jand" suffix. The first idea that comes to my mind is that it might be a deformation of the famous "+Kent" suffix, but I'm not convinced as the k/j sound change is very unusual.
As you mentioned, this word is used just as suffixes which comes with nouns. For instance: Birjand= city of well ( water hole ), Khojand= city of sun, Dastkand= city of handmade, Tashkand= city of bowl or stone,
The letter of j & k are same in pronunciation because of Arabic hegemony.
This suffix must be from northern Iranians because I have not heard it in southern ones. On the other hand, the word "kandan" means dig , therefore it is acceptable that ancient Iranians had used it as a city because we always say "the cities used to be built by digging". Like canal in English.
Although the most scholars believe in Arab domination, this shift in letters is accepted only in written literature otherwise local people never change their ancient spoken literature .
On the other hand, the Persian language has several local dialects in which the people of a special region tend to use a letter more based on their own vocal chords, while in another region they use the same word with a different letter, surprisingly they all understand each other without any problem.
Although Iranian dialects change from region to region, the official language is the same for everyone, and it is the people's vocal cords that do not allow the official words to be pronounced correctly.
For example the word "Red" that is "Ghermez" in Persian but you certainly hear with these pronunciations:
Kermez, kirmiz, etc.
By the way , officially jand and kand are same all around Iran but based on the interests of writers and people in certain periods of time, this word has been changed many times.
Before Arab hegemony, northern Iranians used "kand" as city after different nouns while southern Iranians used "shahr" after nouns to point the specific adjective for a place. Nowadays you can find various cities and villages around Iran that have suffixes like , kand , kendi , jand , shahr , shihir which All of them refer to the word "city.