Salary sacrifice of bonus

Hi all, I have Reckon Accounts Premier. I have been told an employee will get a bonus and wants to salary sacrifice the lot. I do not want the bonus to show in his gross on his payg summary, but the sacrifice needs to be at RESC.

I have to set up the item 'salary sacrifice' - any pitfalls to look out for?

Then how do I process his pay without affecting his gross?

Sorry, I am not very adept at payroll.

Thanks for any help.