rent reduction
A straightforward compound noun used in legal, real estate, and everyday contexts to refer to the lowering of rent.
I negotiated a rent reduction with the landlord.
The application process for rent reduction is complicated.
家賃 is the common word for 'rent' (housing), while 賃料減額 specifically refers to the act or concept of reducing that rent.
減額 is a general term for 'reduction in amount', applicable to various contexts; 賃料減額 narrows it to rent.
Compound of 賃料 (ちんりょう, 'rent') and 減額 (げんがく, 'reduction').