@pauln saying that depending on your card terminal reports it may well report cash back in a seperate total, have seen machines handle tips in different ways similarly, included in total with tips value seperate or a sales/payment total and tips on separate line.
If that the case just noticed you can have multiple translation types on a calculation so you could probably add a second transaction type to have a side account for ‘cashback payments’ to separate from ‘sales payments’ while still ‘returning/refunding’ the amount from cash…