"To write the memo" means to compose a short document that summarizes important information or instructions which will be shared with others in a business or organizational setting. Full definition
In that case, the offer wouldn't be in writing, although you should write a memo for the transaction file with all the offer details. (realtormag.realtor.org)
I don't find this particularly convincing so as far as I am concerned it is still impossible to know who wrote the memo. (planet3.org)
You can use it to do tasks like translate the full sentence, send animated messages or write a memo on the lock screen. (droidholic.com)