Braun Strowman winning the briefcase at WWE Money In the Bank was likely planned a while back.
Dave Meltzer indicated on Wrestling Observer Radio that Braun winning Money In the Bank and possibly cashing in at SummerSlam during the match between Roman Reigns vs. WWE Universal Champion Brock Lesnar may have been hinted at by a WWE source when the Reigns vs. Lesnar plan was revealed last week.
As noted, Reigns is expected to face Lesnar again at the August 19th SummerSlam pay-per-view. The match was not confirmed internally as of last week but that was the plan at one point. This will likely be the end to Lesnar's current WWE run as his contract is expiring.