All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
The fee for a service becomes due upon being given the right to benefit from the service. The Encyclopedia of Fiqh reads, “When the tenant is granted the right to full use and enjoyment of the property, or if the lease period expires and there is no impediment to his enjoyment or use of the property, then the fee for it becomes due because he actually used the property or he willingly did not exercise his right to use it during the contractual period.”
As long as the place where you are studying granted you the right to attend the class but you did not exercise this right because you missed the class, then you are still liable for the tuition fee for the class that you have missed. You are obliged to pay the fee to that woman, and it is impermissible for you to refrain from paying it.
This is in case that there is no condition stipulated in this regard. Since a condition does exist, then you are obliged to pay the fee for the class with greater reason because the condition clearly states that the fee must be paid for missed lessons except in two cases: the student has a valid reason for missing the class or gave a 24-hour notice prior to a session commencing. It seems that neither of these two cases apply to you; hence, you are obliged to abide by that condition and pay for the class. The Prophet, sallallaahu ʻalayhi wa sallam, said, “The Muslims are bound by their conditions, except a condition that forbids what is lawful or permits what is unlawful.” [At-Tirmithi]
The basic principle with regards to the conditions pertaining to worldly interactions and dealings is that they are valid and binding unless proven prohibited by evidence from Islamic law. Ash-Sharh Al-Mumti’ ʻala Zaad Al-Mustaqni’ reads, “The basic principle is that contractual conditions are permissible and valid, whether in marriage, buying and selling, hiring and renting, mortgages, or endowments. The ruling on the conditions that are stipulated in contracts, if they are valid, is that they must be fulfilled...”
Talqeeh Al-Afhaam Al-ʻAliyyah bisharh Al-Qawaaʻid Al-Fiqhiyyah reads, “The thirtieth rule: the basic principle in the area of worship is that (any) conditions are forbidden except if there is evidence in the sharee'ah to indicate otherwise. On the other hand, in the area of transactions, the basic principle is that (any) conditions are permitted unless there is evidence in the sharee'ah to indicate otherwise.”
Allaah knows best.