According to Catalog Sale Items module page is the module compatible with Magento CE 1.2 and 1.3 version.
It looks like it has some dependancy check inside and downloads required files for the module required Magento version ... some modules just have this feature, but I have never seen the downgrade of Magento core files - it usually upgrades only necessary core packages.
My advice would be next time just check the module compatibility with your Magento version and do not install modules not compatible with your actual Magento version. I most cases it would simply work, but in some rare cases it can break your Magento installation just like this module did.
Or backup DB and install the module on your local development Magento copy first and test how does it work, but never go stright to live server ...