Payroll

My employer paid me for staying late on Friday but refused to pay when I returned on Monday to finish my assignment.

Short Answer: No, if your employer knew or should have known you worked overtime, they must pay for it.

Full Answer: Under the Fair Labor Standards Act (FLSA), employers must compensate employees for all hours worked, including unauthorized overtime, if they knew or had reason to know that the employee was working. The employer is responsible for preventing unauthorized overtime and not refusing payment after the fact.
You are entitled to overtime pay if you stayed late to complete work-related tasks and your employer was aware or should have been aware.
To resolve this, you can raise the issue with HR or file a complaint with the U.S. Department of Labor's Wage and Hour Division.

General Federal Standard: Employers must pay for all hours worked, including unauthorized overtime, under FLSA rules.
PAYROLL