As I understand his all post he want to hold bitcoins for three weeks but no one going to do this as he is completely newbie here
But I don't have any possibility to scam or fool the client, in case I do they just get full refund.
Be careful. Even though you may have good intentions, you will most likely get negative trust if you start this service as a newbie.
People don't trust you (even though what you say about refund is valid), and they will make sure that it reflects on your profile. I'd suggest getting a few ranks first.