You should be ok to sand that back. The main problem with oil based varnish is that you need to sand all of it off. If you don't you will see where it has been touched up.
Personally I am a recent convert to the exterior OSMO oils. Provided that the wood doesn't get a chance to discolour, you can just wipe another coat of oil over the top.
I suspect that your door hasn't been sufficiently sealed on the underside. It is quite common given that the decorator isn't generally expected to take the door off to seal the underside, and in reality, most chippies (not all) don't want the hassle of applying a slow drying finish to the underside which may end up rubbing against the carpet.