You cannot convert a string to a single byte, because strings are almost always bigger than one byte (to put it in perspective: one byte can at most encode one character).
If you only want a single byte, convert the string to a byte array and choose one of those bytes, ignoring the rest:
byte tempArray = billingID.getBytes();
byte dealershipID = tempArray;
However, this is almost certainly meaningless. Please have another look at your requirements and rethink this.