Disclosure

I reached an agreement with the HOA for payment of a "Special assessment" this is for one month only can the HOA send a letter to all members telling them of this agreement?

There's really not enough information provided.

Were you behind in making the special assessment payment?
Did the Association want all of it up front and you wanted to make monthly payments?

Perhaps the Board liked the arrangement and wanted to offer it to others.
Perhaps this is an issue where an account is delinquent and the Board wants to keep the members informed of the progress.

In general, as this board has discussed several times in various threads, it's typically the Boards option to release this information or not. This is because, unless the State specifically prohibits it, any member has a right to see the records of the Association. Therefore, the question becomes can the board wait for a request to see the info or should they freely publicize the info. This is a question only your Board can answer and there are pros and cons to doing either one. Personally, I would wait until a request is received from a member. Others, like your Board, might decide to offer the info without being asked.
As I posted, typically, either option can be utilized.