Even the parties negotiating do not have all the information. The purchaser doesn’t know what the seller is willing to accept. They speculate, like people do on this forum.
If Skriniar’s market valuation for Inter was 50-55 million, he would have been sold 100% guaranteed. Considering he has only one year left in his contract and Inter is in need of cash this seems reasonable to the purchaser. PSG may not know that Skriniar is a loyal player who fired his agent in the last contract extension with Inter.
Once PSG understood that 70M was Inter’s maximum, Inter had a 1/3rd chance of selling him. Since 60M would have been done 2/3 of the time.
The fact that PSG stayed in negotiations for the time they did, told Inter that they are still interested in the player and may still meet valuation.
However, at 70 million, PSG may just walk, which at the moment, we can speculate is 2/3 likelier than the 1/3 chance of purchase.