You might be getting the value before it has been transferred to the model

I can map like this:

mMapVoucherPos->addMapping(ui->cbCostCenter->lineEdit,8);

and the combobox in the dialog shows the right value, but I cannot submit to the model!

@Joel-Bodenmann

Not sure what you mean with own model.
When you want that the user is able to add to the combobox only text which follows your specific rules, you may use a QValidator.
Just saw the setModel of QCombobox most likely you need to use one of QCombobox signals (e.g. editTextChanged) and add it to the model through your own slot method. However, never used that yet.