Is it permissible for a driver transporting students to school to receive a full month's salary even though his actual working period is less than a month?

The default rule is that the driver is only entitled to wages for the days he worked. However, if it was agreed that he is entitled to a full month's wage even if he worked less than a month, then he is entitled to it, in accordance with the Hadith: "Muslims are bound by their conditions, except for a condition that forbids what is lawful or permits what is unlawful."