Everything is negotiable. I have always offered based on month I was buying if the points were there.
Same with selling..I paid based on month if all were there.
I just bought and because they had banked 2018, 2019 and 2020, I am reimbursing for 2020.
But, when I bought SSR in 2017 with Dec UY in July, I only offered 6 months dues even though I got all 2020 points.