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.
In the short term it will definitely create panic so that it will put the market back under pressure and this is caused by a bubble that might make the market a little volatile.
But in the long term the theory may be different because bitcoin is not influenced completely by anything and even the market reaction cannot be controlled and occurs beyond many people's expectations.
El Salvador understands what they are doing but in the long term they just want to turn the investment into a higher value and maybe they are also planning their strategy to collect more.
Germany did it and maybe there is a picture of what happened to bitcoin, then it just formed a little bubble that would cause panic in the market in the short term and after that bitcoin found its recovery point again.