Deferring a PhD to pursue a masters after accepting it. Advice?
Essentially, you have to choose between the two; you won't be able to do this the way you propose in your question.

There are usually only a few allowed reasons why a school will let you defer your enrollment: to fulfill a service obligation for a scholarship; to participate in a special program, such as a Rhodes, Marshall, or Fulbright scholarship; or if you have exigent medical circumstances that require you to postpone the start of your studies.

One reason that is generally not allowed is for you to study somewhere else. Just because you'd classify it as a "career opportunity," the PhD program to which you have been admitted will see it differently. They will make you decide between the two programs: if you want to enroll in the master's program, you'd have to give up your PhD admission spot, and reapply for the appropriate term in which you'd be able to start following your master's program.