Can hair transplant damage the existing hair at the recipient area? + Propecia (Photo)

Transplanted hair can potentially damage the existing hairs but the surgeon should be able to plan around it.

You had a large number of densely packed grafts in the recipient area. When everything grows in you should have an impressive result. While it is hard to tell from the pictures because the hair is shaved in the after picture, if you are noticing the front existing hair growing in thinner, it is possible you have shock loss. Luckily that is usually temporary and should start growing back around the time your transplants start growing. I don't believe propecia will help at this stage with the hair in front. It's just a matter of time and letting it heal and grow. 

It is theoretically possible, but if your surgeon knows what they're doing it should never become an issue. Shock loss however is something that happens fairly commonly but often the hair lost grows back. Consult with your doctor about Propecia but yes it does help after a transplant.