We would greatly discourage this line of work—in this era—for the very same reason that you have mentioned.
If the haircut is permissible according to the Shari’ah, then the income would be Halal; and if it is not, the income would be Makrooh.
In regard to the topic of permissible and impermissible haircuts, Hazrat Mufti Rasheed Ahmad Ludhyaanwi V, and Hazrat Mufti Ahmad Khaanpuri (daamat barakaatuhu) have prepared detailed articles outlining the Islamic perspective of haircuts and providing guidelines through which one will be able to refrain from Kuffaar emulation.
Below we will mention a few of the Shar’ee guidelines in regard to cutting the hair as well as some of the important points detailed in the articles of Hazrat Mufti Rasheed Ahmad Ludhyaanwi V and Hazrat Mufti Ahmad Khaanpuri (daamat barakaatuhu). It is essential for one to adhere to these guidelines in order to emulate the Sunnah and refrain from adopting the alien culture of the Kuffaar.
Principally, there are four styles in which people cut their hair:
3.1 The first style is where the entire head is shaved, or all the hair on the head is cut to an equal length. Shari’ah has permitted this style for males. It is reported regarding Hazrat Ali I that he used to shave his head. It is important to note that it is impermissible for a woman to cut her hair or get her hair cut. It is not permissible for a male to get his hair cut by a non-Mahram female.
3.2 The second style is where the hair of one portion of the head (or different portions of the head) is shaved while the remaining hair on the head is not shaved. This style is not permissible. The Hadith has prohibited this and termed it as Qaza’:
عن ابن عمر رضي الله عنهما أن النبي صلى الله عليه وسلم رأی صبيا قد حلق بعض شعره وترك بعضه فنهاهم عن ذلك وقال احلقوه كله أو اتركوه كله (سنن أبو داود رقم: 4195)
Hazrat Abdullah bin Umar I narrates that Rasulullah G saw a child whose head was partially shaved (i.e. a portion was shaved and a portion was left). Rasulullah G prohibited them from cutting the hair in this manner and said, “Shave all the hair or leave all the hair.” (Sunan Abu Dawud no. 4195)
3.3 The third style is where the hair of one portion or different portions of the head is cut shorter while the remaining hair is left longer. Though this will not fall under the definition of Qaza’ mentioned in the Hadith, it will not be permissible as this resembles the ways of the Kuffaar. There are many Ahaadith that prohibit resembling the ways of the Kuffaar. Similarly, just as it is not permissible to get haircuts which are exclusive to the Kuffaar, it is not permissible to get haircuts which are exclusive to the Fussaaq (transgressors), nor will it be permissible for men to get haircuts which are exclusive to the transgressing women. Rasulullah G said: “The one who imitates a nation will be counted amongst them (in the court of Allah)” (Sunan Abu Dawud no. 403)
The esteemed and renowned commentators of Hadith, Allaamah Nawawi V, have mentioned that the impermissibility of Qaza’ is due to one of three reasons:
a) Disfiguring the natural way Allah E has commanded that the hair be kept.
b) Resembling the Jews.
c) Resembling the ways of immoral people.
Hence, we understand that the above three reasons for the impermissibility of Qaza’ are also found in the case where one cuts his hair in different lengths.
3.4 The fourth style is that which resembles the styles and fashions of the Kuffaar or the Fussaaq. This includes the situation where one cuts his hair at an even length, but styles it or dyes it with colors that resemble the ways and fashions of the Kuffaar or the Fussaaq. This is also impermissible in Shari’ah.
Among the impermissible hairstyles are those listed below:
• Dreadlocks (falls under no.4)
• Buzz cut (falls under no.2 or no.3 depending on the length cut)
• Crew cut (falls under no.2 or no.3 depending on the length cut)
• Mohawk (falls under no.2)
• Flattop (falls under no.2)
• Spikes (falls under no.2)
• Ponytail for males (falls under no.4)
In conclusion, if a barber is asked to cut the clients’ hair in a manner which is against the Shari’ah, he must decline.