First of all, conspiracy theories are a contemporary cancer. Centuries ago no one in their right mind would have thought about anything like that.

What we know is that people started complaining when foreign or local currencies started being tampered with or becoming too inflationed. In the Middle Ages, coin mints could be rented to "private investors" which would settle for an amount of pieces produced, aside from being property of a king or prominent city. These occasions might cause fluctuations in the price of the precious metals used to make these coins (mainly silver), which would in turn decrease the value of the currency, since they were merchants that could buy the precious metal from elsewhere and use it to mint currency. For example, in 13th century Venice merchants calculated when to borrow money based on when the German silver would arrive at the Doge's mint to benefit the low inflation.

Simply put, there was no concept of a government capable of the control that we know of today.

Sources:
Persson, K. G., Sharp, P., An Economic History of Europe: Knowledge, Institutions and Growth, 600 to the Present, Cambridge University Press, 2015;
Le Goff, J., Le Moyen Âge et l'argent, Perrin, Paris 2010