In Hong Kong, any agreements involving real estate must be in writing with stamp duty paid. Otherwise, the court does not have to recognize the existence of the agreement.
In short - there is no agreement existed in this case.
However - the easiest (and cheapest) way to deal with your issue is to sign the lease agreement and pay the stamp duty. In that case, both parties are protected by the law. Morally, you will not damage any relationships with your friends (as you will definitely lose a friend if you change the lock).