Can we turn down autonomous wishes to donate anonymously?

Imagine a Samaritan living kidney donor, who some time ago has anonymously donated one of his kidneys to a patient on top of the waiting list. He now contacts the transplantation centre once again, to donate part of his liver.
The Centre, startled by this idea, refers him to the regular screening procedure for all Samaritan donations. It turns out that his wish is well-informed, voluntarily made and that he is competent to decide. We acknowledge that a donor's wish should not be followed in all cases, even though this wish is a clear expression of his own free will. However, a refusal must be based on sound moral reasons and it is less clear what reasons these might be.
We outline the most common arguments for refusal, assess these arguments in terms of strengths and weaknesses, and show which arguments, if any at all, are most promising. We conclude, firstly, that we should only assess risks (which include motivations), not judge relationships, and secondly, that it is not a transplant centre's mission to carry out a donor's life project.