LEO Fair Retirement Act of 2023 This bill makes a series of changes to certain retirement and premium pay calculations for federal law enforcement officers (LEOs). Specifically, the bill provides that for purposes of computing the annuity of an LEO under the Civil Service Retirement System and the Federal Employees Retirement System, any premium pay earned by such LEO in excess of limitations imposed on such pay shall be included in the LEO's average pay, contingent on the payment of a specified lump sum by the LEO to the Office of Personnel Management. The bill allows for a nonrefundable tax credit with respect to such lump-sum payments. The bill also makes postal inspectors, federal air marshals, and other specified personnel eligible for availability pay (i.e., premium pay paid to LEOs who are criminal investigators).