Ex-employee overpayment - payroll treatment

If a company pays their employees mid month (e.g. 2 weeks arrears/2 weeks advance), how should it correct an employees pay if they walk out after being paid having not completed a full month of work?

Example: employee A paid £2,000 (net) for February on 14th February (payslip shows £3,000 gross and £1,000 PAYE/NI). Employee A walks out 15th February effectively having only earned £1,500 gross pay (or £1,000 net) and now effectively owes the company £1,000 (net).

Should the February payroll be re-opened and employee A's gross be adjusted and payslip re-issued showing gross of £1,500, taxes £500, "advance" £2,000 so £1,000 due back to the employer?

Or should a correction be put through on the March payroll?

If adjusting February, does that mean a resubmission of the FPS?

If the overpayment isn't recovered, can the company be chased for the PAYE/NI by HMRC on the basis that the ex-employee has received the money net (irrespective of whether they have earned it)