Is salary an employee agreement?

Unless the employer agrees otherwise, you are an employee at will and can be fired at any time for any reason (unless for an illegal reason such as race or gender).  If your employer agrees to pay you a salary, then that it an agreement.  However, unless it agreed to pay that salary and keep you employed for a certain amount of time, it can reduce your salary at any time.  Of course, if it does this, it would still have to pay the old salary for work already performed at the time of salary reduciton.  I am guessing your situation, but if you are an at-will employee and your employer agreed to pay you at a certain rate of pay (express at the rate of a yearly salary, i.e., $40,000 per year), you employer has the right to reduce your pay on a going forward basis.

Salary can BE in an employment agreement, but it doesn't have to be, and it is not itself an employment agreement. An employment agreement is a contract about the job. Salary is simply what you are paid, and you can be paid a salary w/out a contract.