taking back a pay raise

I would like to know if it is legal for an employer in Oklahoma to give you a pay raise on a signed form with both my and her signatures with the new pay amount and date, then 3 or 4 days later say '' i made a mistake and am not able to give it to you, that maybe later after the 401k is in place, i might be able to give you something''. Is there any thing we the employees can do about this.

sincerly

disapointed and confused