What on-line payment system would you choose for the following
situation:
my web-site intermediates payments between client Anna and client Bob.
These two clients have a contract. Anna is paying Bob for something.
When Bob starts to do what he has to do, my website charges Anna's
credit card. If he does the job right my site will transfer the money
into Bob's account. If he does not do the job, I'll have to return the
money to Anna.