They cannot acquire your domain from you for free unless you are infringing on their trademark(s). Theoretically. But if it is a well known brand, lets say for arguments sake, Amazon.com or Microsoft.com, and you have the domain parked, the parking company will surely provide ads on the domain which infringes on their trademark, and you will be toast, in any court battle. And you could be found guilty under the Lanham Act which can impose fines of $90k per breach (every ad they can show where their trademark is being infringed). In which case it's best to roll-over and give them domain if they come knocking. And finally, you are unlikely to have registered the domain in good faith, if it's a UDRP.
On the other hand. If your domain was BlueBananas.com, and somebody has a trademark for Blue Bananas for a Beach Resort. You could own the domain quite happily, provided you don't infringe on their Beach Resort Trademark. ie for selling bananas which had been dyed blue with food dye.