A decision Donald Trump made in March is proving to be a costly one that has now caused the former president to slip out of Forbes’ rankings for the top 500 richest people, according to a new report. The debut of Trump’s social media company, Truth Social, was a major win for Trump, who found himself much richer as a result. But a “lockup” agreement made sure he wouldn’t cash in his shares instantly, and the wait is now costing him, according to the Washington Post on Saturday. The Post reported over the weekend that the ex-president’s “shares in the parent company of his social media platform…