he obtained the 6-month release letter from "A" to wrok for "B"

after working for "B" for 3 months and he did not like the job --
1. can he go back to "A", in case "A" is ready to take
2. can he go to "C", in case "C" is ready to take him.
in this case, he has to take permission from "A" or
"B"