Out of curiosity and using easy figures...
If a player is bought for 100m on a 5 year contract, his book value diminishes by 20m per year right?
If in his last year, he's worth 20m on the books and you extend for another 5 years, then he'll depreciate at 4m a year right?
But what if you let the full 5 years expire (theoretical book value of 0), then renew. What is his book value deemed to be? (zero?) and is there any sort of 'bridging' contract that could allow a player after 4 years, to agree to renew after 5, but not technically do so (thus being able to have zero book value without running the risk of him leaving on a free)?