Seatpost clamp bolt replacement
As long it secures the seatpost I don't see how a screw instead of bolt can damage the bike. If it fails, the worst case would be a seat sliding down. You might find it inconvient if screw has head that requires tools not usually used for bicycles, like phillips screwdriver or a hex head.

New seatpost collars are cheap, so you can buy a replacement to feel subjectively safer.