I think here you could try some sort of bluff :
Tell them you are very happy with your returns and that you have a lot more money to invest, like 10 times what you invested now. But that you want to be sure withdrawal is possible with them and that you want to see how much time it takes for the money to arrive to your account. So you ask to withdraw your initial deposit, telling them that if it works you will deposit a lot more.
Or you could tell them that you told your family and friends about the awesome returns, but some are skeptical and are asking a proof from you that you can withdraw your funds at anytime before investing their own funds.
Just basically : try to scam the scammer. I think you got “nothing” to lose.
If they refuse the withdrawal, try to negotiate : ask for 80% of the initial deposit instead. And always promise that they will receive a lot more after doing what you are asking.
Try to sound as naive as possible.
If this doesn’t work, than start threatening and report them.
Best luck to you!