Using a variable inside ‘sed’

I have declared a variable in a file. I want to set the value of this variable, to a variable in a different file. When I tried to use ‘sed’ to accomplish this, I encountered an error. The error was, its not getting the value from the old file and just places the variable name in the new file. By changing the command slightly, It works fine. Given below are the commands I used.

OLD-FILE

a=one

NEW-FILE

b=four

$ source OLD-FILE

$ sed -i ‘s/b=.*/b=\${a}/’ NEW-FILE    #Output : b=${a}  #Wrong

$ sed -i ‘s/b=.*/b=’$a/ NEW-FILE         #Output : b=one    #Correct One

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