My employer considers me on a 50 hour salary ,paying 700.00 weekly ,but if I only work 48 or anything less than 50 hours a week they pay me only straight time ,what I'm saying is that if I work 45 hours they pay me 45 hours straight time ,I'm just wondering if that is legal

You are either on salary, or not on salary.

Salary is not a set number of hours. If they deduct money if you work less than 50 hours , then you should be getting over time for over 40. ** assuming this is in the US