If someone spent some money from amanat (trust)?

Suppose a person, have amanat (money) of another person. Now, what is the islamic ruling regarding "if he/she spent some money from it, considering if the one whose amanat he/she has, asks for that, he/she will pay him/her in complete". So, can he/she spent money from another ones amanat and when he gets gets money from say salary and will put that money back into that amanat.