The advance rent was tendered in consideration of your taking occupancy on May 1, and the landlord could not deliver possession on that date, therefore she is in breach and you're entitled to DAMAGES against her for the difference in cost of suitable replacement housing.

Alternatively, landlord's failure to deliver possession destroys the subject matter of the contract, therefore there is no contract and you are entitled to the return of your money based upon the legal theory of quasi-contract, which means that the landlord is unjustly enriched at your expense and you are entitled to restitution of the amount misappropriated.

She never talked to an attorney...in my opinion -- otherwise, he/she would have told her to give the money back.

Oh...okay, that makes you a third-party beneficiary to the contract between the Red Cross and the landlord. The landlord is still in breach and as a consquence, you cannot benefit from the Red Cross' gift of $500 to you. So, you should be entitled to the money so that you can benefit. However, if you sue, you should provide notice of the action to the Red Cross in case it wants to join. Otherwise, you may look to the judge like you're trying to get the money that should be returned to the Red Cross.

I realize that you say that the Red Cross told you that it was between you and the landlord -- but the judge will be suspicious, so that's why I'm suggesting you provide notice -- or get the Red Cross to write you a letter releasing you from liability for the $500 if you collect it.