How can we reconcile jesting with truthfulness and the Prophet’s — may Allah’s prayers and peace be upon him — saying to Zahir, “Who will buy this slave?” when he was not a slave?

The Prophet, peace and blessings be upon him, used to joke, but he would only speak the truth. This is evidenced by what Al-Tirmidhi narrated from Abu Hurayrah. Another example is his joking with Zahir ibn Haram when he embraced him from behind and said, "Who will buy the slave?" By the word "slave," he meant servitude to Allah Almighty. Al-Mulla Ali al-Qari explained that the intent of the question was: who will honor this slave or exchange him?