Not if you assume a fairly free market (not many cartels, etc). If you could buy bread for $10, and now earn 50% more, a merchant could raise the price to $15

Not if you assume a fairly free market (not many cartels, etc). If you could buy bread for $10, and now earn 50% more, a merchant could raise the price to $15 to try to grab the 'newly found money'. But they would be just raising their margin, since the cost of producing the bread is the same. A free market then will self-regulate via competition. A new merchant could enter the market charging $14 for the bread, undercutting the $15 guy. And so on, until the price reaches equilibrium based on what the people are willing to pay. Of course, there's value in the discussion on whether UBI changes the costs of production and/or demand, but it's not as simple as 'income raises, price raises'.