Not much actually.
Bitcoin price will of course dip but not much and it will correct and recover in no time. The news of El Salvador selling its bitcoin and not buying again will be headline news in the bitcoin and crypto world and that will make more people sell their bitcoin in a panic because they're scared that their investments will depreciate or that El Salvator knows something others don't that's why they're selling, but those who actually understand bitcoin won't be moved. They know that bitcoin is not dependant on a single entity and with time, the price will recover, however much it dips.

You can take Germany for example. Germany sold more bitcoins than El Salvador has and they market just bleed a little and recovered.