If the axle was level when they pulled the shaft out, there could be oil residue in the tube. Maybe that is what is running out now. Clean up what you can in the "C", check the diff fluid level, and see what happens after you drive it for a little while longer.
However, it's common for the seal to start leaking after the shaft is removed and reinstalled. It may be due to the installer not being careful, but they have been known to leak even with being very careful.
I just did three sets of ball joints with no leaks afterward.
What I do is jack the axle so the side I'm working on is higher than the differential, thereby avoiding having oil spill into the axle tube.
While the shaft it out, I clean and grease the splines and the smooth surface the seal rides on. I also make sure the little alignment discs are in place on the shaft so as the shaft approaches the seal, it will be at the right height to go into the seal with no damage. Scraping and vacuuming the inside of the axle tube ensures no rust or other debris is pushed ahead of the shaft and into the seal.
Not many paid installers will go to those lengths to avoid post-ball joint leaks.