Actually Im not sure about the answer but in very old time Iran was called ((Pars)) and its language was ((Parsi ))but after the Arab language inter to Parsi language because Arabs don't have P in their alphabet so then Parsi changed to Farsi.

''Persian is the English version/translation of Farsi. So, it makes sense to call it Persian when we speak English. How would we then call the other two variants of the same language, namely Dari and Tajiki? If we label the three as Persian, Dari, and Tajiki, we will miss the tight genetic relationship between them: they are all derived from one single parent, and are linguistically variants of the same language. They are definitely not three different languages. One solution is to employ Persian for Farsi, and call the other two variants TajikiPersian and Dari-Persian. This type of labeling clearly shows the relationship between the three variants, given the fact that all three variants are derived from Middle Persian or Farsi-ye Miyane. But wait, the unmarked usage of Persian, referring to Farsi, gives the impression that this variant is the main version, and the other two have derived from it. This is not true linguistically though: the syntactic properties of Tajiki and Dari, for example, are closer to the syntax of the earlier stages of Persian than Farsi is. A second solution is to follow the existing tradition used for other languages: the two major variants of Portuguese, for example, are called European Portuguese versus Brazilian Portuguese. British English versus Standard American English is another example. Given this analogy, we will arrive at Iranian Persian (for Farsi), Tajiki Persian, and Dari (Afghani) Persian.'' Maybe this link could be useful for you too: https://www.quora.com/Why-do-some-people-prefer-to-call-Farsi-Persian-or-vice-versa

Actually Im not sure about the answer but in very old time Iran was called ((Pars)) and its language was ((parsi ))but after the Arab language inter to Parsi language because Arabs don't have P in their alphabet so then Parsi changed to Farsi.