Allowed inputs for field "Probablity" is hardcoded and related to SD document type. For sales order (Document type "C") this field is Display only and %100 always. For inquiries ("A") or quotations ("B") you can change its value.
A solution recommendation: Try changing the document type from C to B, press enter, then change probability, and change document type to C again. It seems like as if it will work, but I'm not sure.