I don't know why some people are always so insecure to prove that Islam is fantastic. In the process this kind of silly items pop up.
Instead of just saying the shop keeper was such as good man that he offered money and help to even the robber, adding the bit about the robber wanting to convert (which he obviously did not since he ran away the first chance he got) simply makes the noble act of the shop keeper less noble.
Since when talking about goodness of anything is insecurity?
Pretty lame argument you have there.
Just swallow the information and take an antacid while your are there. :-)