Is it permissible to stipulate a fixed minimum monthly invoice in a freelance contract, regardless of the actual hours worked, with an increase in the invoice if the work exceeds this minimum?

There is no objection to stipulating the aforementioned wage, which is $3200 for 16 hours a week, and $50 for each additional hour; because the basic principle concerning conditions is permissibility and validity. Only what the Shari'ah has indicated to be forbidden and invalid is prohibited and nullified. Sheikh Al-Islam Ibn Taymiyyah stated that the basic principle concerning contracts and conditions is permissibility and validity, and that "conditions which do not contravene the Shari'ah are valid in all contracts."