PAYE on Final Payment to Employee leaving to work overseas

I have processed a final one-off sales commission payment for a higher-rate tax paying ex-employee who has left to start his own business in the USA. I processed it as an irregular payment in August 2018, with no reissue of the P45. Sage Payroll has deducted PAYE at a rate that would be reasonable on the assumption that the employee will continue in employment in the UK (31.6% of the gross payment). The employee has queried this, as he will have no further income subject to UK tax in the 2018/19 tax year. 

Is there a way to roll back and rework the PAYE calculation with assumptions more in the employees favour?

If not, it looks like I will simply have to advise them to claim a tax rebate from HMRC. Does HMRC have an office that can expedite this?