what you mean "提到过我们将保留...."

Just verbal?  Do you have anything in writing?  in E-mail?  recorded conversation?

You must understand verbal agreetment has no leagal stand, basically means nothing. 

As to the trouble, you can be sued for unlawful withholding tenant's property, like JY said. 

For this one, they already moved out so probably no more trouble for you unless you create it.  But for your future rental management, the way you thinking is really dangerous.