Is it permissible in Islamic law to leave a girl one has loved for a year due to the unavailability of sufficient financial means for marriage, or is that considered a sin?

If the reason for breaking off the engagement is an inability to provide for the wife, then your action is correct and appropriate, because it is forbidden for one who cannot provide for his wife, or who would provide for her from unlawful means, to marry unless he fears committing zina. However, if you were capable and promised the girl marriage, then it was befitting to fulfill the promise, even though abandoning it is permissible, as fulfilling a promise is recommended and not obligatory according to some scholars.