I've been taught that under Saint Stephen and even under Coleman the Bookish, Hungarian serfs were expected to pay not a fixed sum of currency to their landlords, but 1/10th of their harvest, and offer another 1/10th to the church.
I was also taught the serfs did not really use currency, and relied mostly on barter within their village to get their things.
Now, I understand how a baron or a priest might collect their taxes, but how does said baron or priest turn that pile of grains and other perishable goods into currency to pay as a tax to their own sovereign? Who do they sell it to?
I am not a credited historian, but I have had similar questions. Using some Reddit fu I have the response in How did taxes get collected and make their way from the source to nobles and kings in medieval kingdombs (especially England and France)? , What prevented people from selling under the table in medieval times?, How/how much were people taxed in medieval/early modern Europe?,
A summary of these is at the basic level, Barons or priests (Hereafter referred to as lords) don't have that pile of grain they need to turn into currency. Often it was easier to outsource the whole problem to others, or the tax farmers. In tax farming, an individual pays the lords a set amount of coin and in return is responsible for collecting the grain and arranging for it to be sold to offset the fee paid to the lord.
In regards to who they sell it to, I imagine it was whoever was willing to buy, major cities, grain poor regions, etc. Medieval farming practices didn't lend themselves to having a surplus, so they were often literally one bad harvest away from famine.