i agree with quik, but the one case that should have contracts allowed under the current game dynamics are trades in season. so one can trade for a player in the last yr of his contract.
but i presume this contract stuff is also in place to enforce a realistic game dynamics, since in the NFL players with 1-yr left on the contract are rarely traded or extended during the season. there's perhaps 15-20 extensions during the season, with 3-4 high profile transactions late in the season, such as marcus peters, whitney mercilus, devante parker, lane johnson, and shaq thomson last yr.
NFL Transactions | Spotrac
here's jim's blog post on the issue for those that haven't seen it.
Renegotiations in Front Office Football – Football Frontier