I think the team can say no to the $20M posting fee and keep him. They expected much more and may try and send a message by refusing the $20M, but $20M is a lot of beans for a Japanese team to turn down.
The entire payroll of the Rakuten Eagles is about $17.49 million depending on the exchange rate
The salary for Tanaka is about $3.89 million
Tanaka represents 22.24 % of their entire payroll and makes 4x more than any other player on the team.
That means that the $20 million posting fee could pay the entire payroll of the team for close to the next two years without Tanaka since average salaries dropped by 4% last year over there. I'm not sure the Rakuten Eagles will want to risk that, but if they did it would be out of protest for what they feel is getting cheated out of an extra $40 million.