As many of the member said it's HALAL and the main reason is if it's acceptable from the 2 sides then its just a money as they do back in years " Deal is accepted between two people without any condition stumbling the deal and the agreement is clear and didn't effect the Sharia, it's Halal"
I remember I have long talk with a lot of Mash'aikh around me and some of them didn't understand what is actually Bitcoin and while explaining to them what exactly it is, they gave me the exact answer I just mentioned above.
Hopes that helps you.