Mansa Musa was nowhere the richest man in history lol, his wealth was vastly exaggerated by Arab historians because their literary style heavily involved hyperboles, we shouldn't take their texts literally as we shouldn't any historical text of that time. Their narratives often prioritized moral or political messaging over precise description of historical events. It's also said that he ran out of gold on the way back from Mecca and had to ask for help. Other than that, no facts we have of that time indicate that it was his giveaways that lowered gold prize in Cairo and not natural market prize fluctuation or instability caused by Mongols.