The mass-metallicity relation (MZR) represents one of the most important scaling relations in the context of galaxy evolution, comprising a positive correlation between stellar mass and metallicity (Z). The fundamental metallicity relation (FMR) introduces a new parameter into the dependence, namely, the star formation rate (SFR). While several studies have found that Z is anti-correlated with the SFR at a fixed mass, the validity of this statement has been questioned extensively and no widely accepted consensus has been reached thus far. With this work, we investigate the FMR in nine nearby, spatially resolved, dwarf galaxies, using gas diagnostics on integral-field spectroscopic data of the Multi Unit Spectroscopic Explorer (MUSE), pushing such investigations to lower galaxy masses and higher resolutions. We find that both the MZR and FMR exhibit different behaviours within different star-forming regions of the galaxies. We find that the SFR surface-density-and-metallicity anti-correlation is tighter in the low-mass galaxies of our sample. For all the galaxies considered, we find a SFR surface-density-and-stellar-mass surface-density correlation. We propose that the main reason behind these findings is connected to the accretion mechanisms of the gas fuelling star formation, namely: low-mass, metal-poor galaxies accrete pristine gas from the intergalactic medium, while in more massive and metal-enriched systems, the gas responsible for star formation is recycled from previous star-forming episodes.