The Elders letter you just sent out says that the Congregation Elders need to get together to decide if the suggested amount is okay or whether they need to lower or increase the amount. Once that is decided, they need to get a resolution from the Congregation for that amount. The wording in the letter states that the money will come from monthly donations made to the local Congregation.
Because the Elders are required to get a resolution from the Congregation, this now obligates them to donate that stated amount. I agree with the reddit poster, it becomes mandatory (as a Congregation).
There is no date for how long this resolution is to last and there is no provision for the Elders to review that amount.