Esmeralda entered into a friendly agreement with Roger covering Esmeralda's lot covered by TCT No. 5678 located in Makati City with an appraised value of PHP 6,000,000.00. As stated in the friendly agreement, Roger was allowed to enter the property to start his food business after paying PHP 600,000.00 as down payment with the condition that the deed of absolute sale shall only be executed after paying the full price of PHP 6,000,000.00.

Unknown to Esmeralda, Roger obtained a loan amounting to PHP 5,000,000.00 from Silver Bank payable in installment for two (2) years. To secure payment of the loan, Roger executed a real estate mortgage (REM) over the same lot covered by TCT No. 5678.

Prior to the approval of the loan, bank loan appraiser Mrs. Zenaida conducted an inspection and verified that Roger was indeed occupying the subject lot.

After a year from executing the Contract to Sell, Roger failed to pay the amount of PHP 6,000,000.00 despite Esmeralda's written demand. At the same time, Roger failed to pay his installment for the loan with Silver Bank despite several demands.

Silver Bank filed a case for foreclosure of the mortgage property covered by TCT No. 5678. Esmeralda intervene in the case claiming that she is the owner of the mortgaged property. Silver Bank argued that the Roger was already the owner of the subject property since the "friendly agreement" should be considered as a valid contract of sale.

Given the foregoing facts, Answer the following below:

a. What is the nature of contract between Esmeralda and Roger? State your legal basis.

b. What are the requisites of a valid contract of sale?

c. Was the real estate mortgage (REM) valid? State your legal basis.

d. Decide the case with legal basis.