Saying Allah Hafiz Instead Of Assalamualaikum
Not every instruction given by the prophet or Allah is must or obligatory. Here a manner is being taught. To be a sin is something unethical or immoral or violation of an obligatory act. Having said that it is not advised to say salam twice in a single sitting or meeting. So if one says salam once and then on leaving one says another prayer there is nothing objectionable. Religion is simple and easy. This is its beauty.