למלך את שבתותיה
To the king her sabbaticals.
An explanation of Rashi, channeling Mizrachi:
and thus appease [This verb is in the causative form, meaning: The Land will appease] the King regarding its sabbaticals (Mizrachi).
Shadal comes to the rescue with a simple girsological correction, based on a manuscript of Rashi he had in his possession:
Thus, the word is not למלך but למלאת, to repay. And Rashi is explaining the meaning of the strange word וְהִרְצָת here. It is still indeed a hiphil.
A bit later, on pasuk 41:
Thus he explains that the word really means to appease. The meaning of הרצה is שילם, to repay, but by way of "making peace" (shalom). The idea is that when they humble their hearts, then the repayment of their sins will be finished. And so too the land, it is as if it is obligated to pay to Hashem its sabbaticals.