Is money earned from completing office work from home instead of the main workplace, after finishing field work early and saving travel time, permissible or forbidden?

The duty in work is to implement the texts of the contract. If they obligate the employee to remain at the workplace throughout official working hours, then it is obligatory to remain, based on the words of Allah Almighty: "O you who have believed, fulfill [all] contracts." And a private employee, like a salaried worker, is hired for a specific duration and is obliged to remain at their workplace throughout the period of hire, unless permission is granted by someone authorized to give it.