Advice based on law principles

In March, Dahlia rented a room in a certain complex for $1000 a month.
Having heard of her noble intention to pursue studies overseas and her
financial difficulties the landlord, Diamond, agreed to accept $500 in rent
for the next three months (April to June). She paid $500 in April. At the
beginning of the second month (May), Diamond informed her that the rent
will return to the full amount from this month (May) and she would have
to pay the other $500 for April. She refused. Advise Dahlia