Summary:
* I was seeking to tumble BTC acquired from coinbase with bestmixer.io (about $260)
* I inadvertently sent the coins to the bestmixer.io public address and not the designated (transaction-specific) mixing address in the instruction.
* When I discovered this and identified the transaction, Bestmixer.io said they can only give the funds back if I can prove that I was the one that sent the coins.
* To do this, they said I need to digitally sign a message to them with the same address originally used to Send.
* Unfortunately, I cannot do this with the GreenAddress wallet.
* Per their FAQ the GreenAddress wallet uses a multisignature format; it does not permit re-signing of a Send transaction, and each new transaction creates a new address
 
Questions:
* Might there be a way for me to digitally sign the prior Send Transaction?
* Might there be any other way for me to prove that I (and not someone else) initiated this prior Send transaction?
* Any other ideas that might help?

Offer:
I'm happy to pay someone who can recover the funds a 25% recovery fee (about $65)
Please post publicly or PM me if you think you can help.
I will then share additional information in private.