You can use the visa from Employer A to reenter the U.S. as long as the visa is valid. You should take the new H-1B approval notice and the most recent pay stubs with you when you leave the U.S.
This answer is of a general nature and should not be relied upon as final, nor is it intended as legal advice. Consult with a qualified attorney before making any legal decisions. Gen Kimura, (832) 247-6932.
AC21 permits this
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