Does Hijamah Break the Fast?
Question

I am a woman who does cupping for other women. As far as I know, cupping breaks the fast of Ramadan, because the Prophet (blessings and peace of Allah be upon him) said: “The cupper and the one for whom cupping is done both break their fast.” 
But what if I am not fasting for a valid shar‘i reason, and a woman who follows the Shafi'i view (which says that cupping does not break the fast) asks me to treat her? Am I sinning if I do cupping for her when she is fasting?
Praise be to Allah.Does hijamah break fast?
The fuqaha differed as to whether cupping (Hijamah ) breaks the fast of the fasting person or not. There are two well-known views:

The first view is that Hijamah breaks the fast of the fasting person . This is the view of the Hanbalis and a number of the early generations (salaf). They quoted as evidence the words of the Prophet (blessings and peace of Allah be upon him), “The cupper and the one for whom cupping is done both break their fast.” (Narrated by Abu Dawud (2367) and Ibn Majah (1679); classed as sahih by al-Albani in Sahih Abi Dawud (2047)

This view was regarded as correct by a number of scholars, and fatwas to this effect were issued by the Standing Committee and by Shaykh Ibn ‘Uthaymin (may Allah have mercy on him), who also issued fatwas to this effect on our website.

The second view is that hijamah does not break the fast of the fasting person. This is the view of the majority of fuqaha.

Which school to follow?
If you follow the view that cupping breaks the fast, then you have no right to treat a fasting woman with cupping, even if she follows the view of scholars who say that cupping does not break the fast, because you would be doing something that breaks the fast according to your belief, and breaking the fast of a fasting person – with no valid excuse – is not permissible. Therefore this is regarded as being a direct cause of doing something that is forbidden.
The fuqaha have mentioned a similar case, which has to do with someone who thinks that a particular action is permissible, helping someone to do it who believes that it is forbidden, or vice versa.
Ar-Ramli said in Nihayat al-Muhtaj (10/217): 
“If one who believes that it [playing chess] is permissible plays with someone who also believes that it is permissible, [then that is permissible], otherwise it is forbidden, as was regarded as being more likely to be correct by as-Subki, al-Adhra‘i, az-Zarkashi and others. And the reason for that is clear, because he is helping him to commit sin, as we believe that it is binding upon the other person to adhere to the view of his madhhab,
And because ash-Shafi'i regarded it as essential to tell him not to do that, because of what we mentioned above, that anyone who does something that he believes to be unlawful must be rebuked for his action, even if that rebuke is delivered by someone who believes the action to be permissible.”
 And he said in his commentary on Asna’l-Matalib (3/343): 
“If he thinks it is prohibited or he plays with someone who thinks it is prohibited, in that case he is helping him to transgress what he believes to be a prohibition, and this is heedlessness.
As-Subki said: A similar case is if two men get involved in a business transaction at the time of the adhan [for Jumu‘ah], and attending Jumu‘ah is obligatory for one of them, but not for the other one [e.g. because he is a traveller]. The correct view is that it is forbidden for both of them (to carry on with the transaction), and al-Adhra‘i, az-Zarkashi and others agreed with this view.
But the issue we are discussing here is more subtle, because engaging in a business transaction with one for whom Jumu‘ah is obligatory is something that is well known to us and to him, but the prohibition on playing chess is something that is controversial. Rather what is clear is that what is forbidden is playing chess if one believes it to be forbidden.”
What is the ruling if Hijamah can’t be delayed?
But if cupping is done to treat sickness that is present, or that is expected to occur if cupping is delayed, and it is not possible to delay it until after iftar, then this is an excuse which allows breaking the fast, and there is no blame on you for doing cupping in this case, because it is helping someone in a case where breaking the fast is permissible.
But if it is possible to delay it until after iftar, or it is not to treat sickness that is present or expected to occur if cupping is delayed, and it is rather the regular habit of the person who is being treated with cupping, then in this case it is not permissible for the cupper, if he believes that cupping breaks the fast, to do cupping for someone else.
And Allah knows best.