Please answer it using IRAC structure please

Question 5 David lives in an apartment that he started renting 3 years ago. He pays $1000 per month. The current contract will end in 18 months. When David gets out of hospital he tells his landlord he may have a problem paying his usual rent of $1000 per month as he cannot work and get his usual salary. The landlord, Sisca, tells him that she will let him pay $400 per month for the next 3 months. David is very happy and gives Sisca $400 per month for the next 3 months. After 3 months, David goes back to his usual work and gets paid. He gives Sisca $1000 for the next month's rent. Sisca says "oh I am so happy you are back working as usual. Thanks for the normal rent again. But what about the $1,800 you have not paid for the last 3 months (3 x $600)? Please pay me the $1,800 by the end of this week." David is not happy as he thinks it would be unfair if Sisca breaks her promise about accepting $400 per month for 3 months. Advise David if, under promissory estoppel, the court would stop Sisca from breaking her promise about the reduced rent. Issue: Will Sisca be estopped from breaking her promise or does David have to pay the $1800? RL&A C