Advacne received in june-17 and billing in jul-17

A is service provider on monthly basis , A has taken advance of 1 lac from its client  in Jun-17 ( Service tax  law) and for which A is supposed to raised the bill in Jul-17 ( GST law).

Question : In Jun-17 it self  A is paid the service tax  @ 15% on advnace receipt of 1 lac, weather at the time of raising  invoice ,A is also liable to pay GST  @ 18%,?, if yes ,what of the service tax paid @ 15% in the month of june and what is the treatment to be given. in this kind of case.