Thx for your reply, Uncle Gizmo!
Your assumptions are mostly correct. The option group is linked to the field called Validity. The validity field is one of the fields of the Quotation table. And please forgive my English and see if I understand correctly. Do you mean that I need to discard the original option group and just remain the text field for custom value input? And that text field should be bounded to the validity field? If so, this is not what I want. I do want to remain the option group (30,60 and 90) cos they are the usual options to be chosen. But the text field is for exceptional cases if happened.