slavic volxu

Piotr said in a previous mail, no matter which the final origin of the
word should be, then it belongs to the early borrowing in proto slavic,
thus it was developed/changed as the slavic native words.

The only problem I see is the "x".
If the ultimative etymology of the word remains this celtic "volcae"
then the change c > h should have been somewhere in 300 BC and remained
in this stadium until the slavs got it ( when? ).Which arguments are for
sustaining that the slavs loaned it from a german word with "x"?What
speaks against the idea slavic did not changed the "c" in "x" by itself
?