Low budget fairy tale about a princess and a marble

I'm looking for the title of a very very low budget movie from the 80s.

In the Middle Ages a hustler arrives to a village's market to sell a healing potion. His partner, blind and lame, tries it and immediately heals and everybody wants to buy it. Some guards take them but they discover is not because of the scam, it's because they want to bring them to the king. I don't remember the rest, but there's a girl who may or may not be a princess. Some evil character devises a test: make her sleep over dozens of mattresses and put a marble underneath. A true princess is so sensitive that will feel it. The test is designed to be impossible, but she does feel uncomfortable, proving she is a princess.

It's so low budget that it looks like amateur theater, everything is shot on studio.

I don't know which version you saw, but this is a retelling of "The Princess and the Pea," which has been adapted many times in one form or another.

if it's true than just in case
youtu.be/PjnEB1ot3SQ?t=2
http://www.imdb.com/title/tt0197781/...ef_=tt_trv_cnn

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Thanks, it's this one. But, like often times, I think I glued together two different stories. The first part of the plot I wrote has the same tone and choreography that this Faerie Tale Theater, but I checked and is not part of this or another episode in the series.