What is the ruling on the Witr prayer in which the worshiper intended to pray two rak'ahs (shaf') then, out of forgetfulness, said the taslim with the Imam? And is it permissible for him to pray the Qiyam (night prayer) afterwards? And is he considered to have prayed with the Imam until his departure by doing so?

If you intended to pray the shafʿ (even-numbered units) with the Imam and then gave the taslim (salutation of peace) forgetfully, then the witr (odd-numbered unit) has not been performed. It is permissible for you to pray the witr again. You will receive the reward for performing qiyam al-layl (night prayer) for remaining with the Imam until he departed, and this reward is not dependent on the witr. You may then pray as many supererogatory prayers as you wish, and then repeat the witr because it was invalidated. However, if you prayed the witr correctly with the Imam, then pray as many supererogatory prayers as you wish and do not repeat the witr.