Well, you'll need to be more specific than 'medieval times' if you want a detailed answer. This is a thousand years of history and with great geographical variation. Also, the development of legal and documentary systems played a huge part in how historians can trace what was going on. Immovable property, whether castles, manors, mines, mills, etc. with appurtenances, were exchanged for money, services, goods, or protection by agreement between two parties and with another party bearing witness. The nitty gritty is different in France compared to England, or in Wales compared to England, or in the ninth-century compared to the thirteenth- through fifteenth-centuries. But at the heart of any agreement in the Christian West was a contract (not necessarily a written one) between two parties. I'm not a legal historian and would not claim to be able to discuss the whole spread of medieval property law so if you can pick a period and region I know something about I'm happy to elaborate, when I get a chance.