What is the ruling of Islamic law on taking 25% of the value of an agreed-upon contract with a client, in the event that the client فسخ (annuls/terminates) the contract, which includes this condition?

If you agree with the student on specific hours, then you are entitled to the wage simply by making yourself available for work during these hours. This is because ijarah (leasing/hiring) is one of the binding contracts that neither party can unilaterally terminate without the other, even if there is an excuse according to the majority of scholars. Thus, the tenant's inability to travel or his illness does not grant him the right to terminate the contract. If the hired person has the right to demand the full wage, then there is no harm if he takes only part of it.