There is no discrepancy between what is recorded in Exodus xii. 40 and Acts vii. 6. The former, which speaks of 430 years, includes the time right up to the giving of the Law from Sinai, as is made quite plain in the Septuagint Version, which reads:
“Now the sojourning of the children of Israel in the Land of Egypt, and the Land of Canaan, was 430 years “.
The Samaritan Pentateuch has the same reading.
Stephen, in Acts vii., simply says how long the children of Israel were in bondage and evilly entreated by the Egyptians. All this is borne out by Paul, in Galatians iii. 17, where the giving of the Law from Sinai is expressly associated with the 430 years.
Read “Hebraikon Chronikon”, by Dr. Thomas.