"Come on now Joker, don't be too hasty. I'll call off the men outside them, but you have to ease your hands off of that detonator first. Here, as a sign of goodwill, I'll even put down my shield nice and easy so I can't throw it at you. Everyone knows I don't lie, right?" 

Cap spoke calmly and slowly so that his commlink to the SHIELD agents outside picked up every word and made it obvious he was doing so, then slammed his own shield into the floor of the warehouse so it embedded an inch or so into the floor by his foot to prove he wasn't going to try anything. Not that he wasn't confident that he could take the Joker and probably survive any risk of explosion, but the SHIELD agents outside might not be so lucky, and given the amount of chemicals stored in the warehouse, the people of Gotham wouldn't be either if the explosion vaporized any of these chemicals and put them in the air.

Not that Rogers was entirely a fool either. If it so much as seemed like the Joker was about to push the button, he'd already positioned and stabilized his mighty shield perfectly to kick it from it's upright position on the floor right at the Clown Prince of Crime's wrist and knock the detonator from his grasp with all the speed that his peak human reflexes gave him. It wasn't technically a lie if he kicked it instead of throwing it.

It also helped that the initial plan to begin with was for Steve to come in loud and noticeable while some of SHIELD's best agents snuck in around the back as support, with the perimeter secured as extra insurance. Now to hope the 'best agents' in question were actually listening in and had managed to infiltrate the building. He only said he'd call off the men [i]out[/i]side after all. 

If they hadn't gotten in yet, well sometimes the old ways were best. "So Joker, I have to wonder now that you've got me trapped with pounds and pounds of explosives, what is all of this [i]for[/i] anyways." If villains today were still anything like the ones from the 40s, then they could never resist a monologue.