Wondering if reducing my wage 2.50 per hour is legal after a verbal agreement of keeping higher wage

Did you have a term for how long you would receive the higher wage?  If not, your employer can terminate that agreement at any time, just as you can by demanding a higher wage or you will quit, or simply by quitting.  If you had an agreement that you would receive the higher wage for a definite period of time, you would still have a problem, because in the jurisdictions in which I practice (I believe it is the same in Illinois) a verbal agreement is not valid if it can't be fully performed within a year, so that if you had an oral agreement to receive the higher wage for 2 years, that would probably not be enforceable.  There are some twists you could try.  For example, if the agreement was for you to receive the higher wage until you voluntarily left your employment, you could argue that, since you could have left your employment earlier than one year, the agreement COULD have been perforemd within one year and therefore is enforceable, but it is doubtful if it will work, particularly since your boss is likely to deny that you had any such agreement.