What is the ruling on a dream in which the dreamer saw himself going to 'Asr prayer and being told that his daughter would be named Iman two months before her birth, even though the doctors did not know the gender of the baby, and despite that, her father named her Lujayn?

Visions do not form the basis for binding legal rulings; they do not obligate something that was not obligatory, nor do they prohibit what is permissible. A righteous vision is among the glad tidings of prophecy, as stated in the hadith (definition): "Nothing remains of the glad tidings of prophethood except the righteous vision, which a Muslim sees or which is seen for him." Therefore, your vision that someone had a daughter named Iman does not compel him to name her with this name, even though it would be preferable for him to do so.