Creating the same unique ID in multiple tables
Add EmployeeId  to columns to both tables.  Populate the master EmployeeId  with the query you have, and join the transaction details table to the employee master on EmployeeName  and lookup the EmployeeId .

eg

with q as
(
   select e.EmployeeId master_EmployeeId, t.EmployeeId
   from HREmployeeMaster e
   join EmployeeTransactions t
     on e.EmployeeName = t.EmployeeName
)
update q set EmployeeId  = master_EmployeeId;