Rafi bin Khadij reported Allah's Messenger (sallallaahu alayhi wa sallam) as saying:
The price of a dog is evil, the earning of a prostitute is evil, and the earning of a cupper is evil.
Sahīh Muslim Hadīth No. 1568
Based on this Hadīth, the Hanābilah said that one is not to take money for offering cupping services. This is one view among the Hanbali school of thought.
The majority of scholars which includes the Hanafi, Shāfi'i, and Māliki schools of thought said that it is permissible for a person to take money for cupping.
They based their opinion on the following Hadīth.
Ibn Abbas (Allah be pleased with them) reported that Allah's Messenger (sallallaahu alayhi wa sallam) got himself cupped and he paid the cupper his charges and he put medicine in his nostrils.
Sahīh Muslim Hadīth No. 1202\
They argue that:
وَلَوْ عَلِمَهُ حَرَامًا لَمْ يُعْطِهِ وَفِي لَفْظٍ لَوْ عَلِمَهُ خَبِيثًا لَمْ يُعْطِهِ. وَلأَِنَّهَا مَنْفَعَةٌ مُبَاحَةٌ فَجَازَ الاِسْتِئْجَارُ عَلَيْهَا كَالْبِنَاءِ وَالْخِيَاطَةِ، وَلأَِنَّ بِالنَّاسِ حَاجَةً إِلَيْهَا وَلاَ نَجِدُ كُل أَحَدٍ مُتَبَرِّعًا بِهَا، فَجَازَ الاِسْتِئْجَارُ عَلَيْهَا كَالرَّضَاعِ.
Had the earnings of the cupper been harām the Messenger of Allāh sallallaahu alayhi wa sallam would not have paid him. Additionally, it is a permissible action which benefits, and so, it is permissible to charge for it.
People are in need of cupping and not everyone is willing to volunteer to perform it. Therefore, it is permissible, according to them, that one charge for it just like one can charge for suckling (a baby).
And Allāh knows best.
The Kuwaiti Encyclopedia of Fiqh