Is it forbidden to tamper with the scales to take dues that have not been fulfilled?

It is not permissible to give short measure or cheat in weighing, nor to transgress the rights of people and consume their wealth unjustly. It is also not permissible to take your companion's money merely because he promised you something and then broke his promise. You must return what you took to him. If you have a legitimate right against him, then demand it from him through lawful means.