(1) Here is a very concise summary of the old system. The Nippon Pro Baseball team sets a "release fee," max $20M, and if more than one MLB team is willing to meet that fee, then it can negotiate with the player's agent on a contract. Bidding wars for salaries could result.
IIRC correctly, Oakland once posted the fee for Iwakuma and then offered him a pittance, sending him back to Japan, so the M's wouldn't get him.
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(2) They're haggling the new deal, which isn't very clear to me. The key entities are Yomiuri and Softbank, which would rather the posting system not exist at all and so work for smaller fees; the rest of the NPB teams; the MLB; and the players' union which (I think) would like to see the posting fee be simply 15% of whatever the player can get on the FA market.
Or not ...