As Future proved unable to pay outstanding dues and losses in its retail operations swelled, Reliance faced "compelling circumstances" and decided to exercise its legal right to take over the stores, the letter added.
Neither Reliance nor Future immediately responded to a request for comment.
Future, which is staring at bankruptcy as its losses grow, has previously called Reliance's move "drastic and unilateral".
Before Amazon blocked it, Reliance, led by India's richest man, Mukesh Ambani, had proposed a $3.4-billion deal to buy Future's retail, wholesale and logistics operations, as well as some other businesses.
But following Reliance's abrupt takeover of its stores, Future sought several assurances in a March 2 letter, also seen by Reuters, asking if Reliance would stick to the deal without changing its value or terms.