What is the ruling on someone who committed an act believing it to be forbidden, but then it became clear that it was not? Is he sinful due to his belief, or not? And if a person believed something but then said the opposite, and what he said matched reality, is he considered a liar or truthful?

One who performs a permissible act, believing it to be forbidden, incurs sin due to their intention and their audacity in committing what they perceive as forbidden, not due to the act itself. This is between them and their Lord, though their sin is not like the sin of one who actually commits a forbidden act. As for the rights of people, the reality must be considered. So, one who has intercourse with his wife, believing her to be a foreign woman, disobeys Allah. No legal punishment (hadd) applies to him, nor will he be punished in the Hereafter with the punishment of an adulterer; rather, he will be punished for his audacity in defying Allah's commands. Similarly, one who speaks what is true while believing it to be false incurs sin for the same reason.