In the After-the-Fact Payroll window, a gray cell can't be edited. Some reasons why include:
Certain cells contain fixed rate taxes, such as Medicare, Social Security, and state disability. QuickBooks calculates these types of payroll taxes based on a fixed percent of gross wages, per state and federal tax rates.
If the amount that QuickBooks calculates doesn't match the amount your client provided, most likely your client used a different method of calculating these taxes than that used by QuickBooks. Adjust the paycheck as you normally would when you find this kind of difference in after-the-fact payroll.
Diff +/- and Calculated Net Pay can't change because they are intentionally calculated by QuickBooks based on amounts in the paycheck.
When entering paychecks for multiple employees in the same instance of the window, not all columns apply to every employee. If a column is for a payroll item that applies to one employee but not another, the cell is gray for the employee that doesn't have that payroll item in their employee record.
How QuickBooks calculates payroll taxes
View and edit details of after-the-fact paychecks
After-the-fact payroll overview