7. To say that something is not necessary, use needn't or don't have to, not mustn't.
You needn't pay now; tomorrow will be OK. (NOT You m̶u̶s̶t̶n̶'̶t̶ ̶p̶a̶y̶ ̶n̶o̶w̶; tomorrow will be OK.)
I don't have to wear a tie at work. (NOT I m̶u̶s̶t̶n̶'̶t̶ ̶w̶e̶a̶r̶...)
For more details, see Practical English Usage 3rd Edition 366.