When a trade cannot go through, especially in Salary Cap Leagues, we hit "Post" or "Accept" and hope for the best. The data exists in the matrices to give us the reason our trades did not go through. If it's a matter of Roster Space or Picks, a message could pop up saying "Your roster is full at this point" or "You no longer have 'X Player' on your roster / the 3rd Round Pick for 2018.' If we are in the midst of multiple trades and accept one, a one line error dialogue box would clear up confusion. In the cases of multiple picks for the same Round in the same year, something to differentiate between them in the Front Office would help.

My biggest suggestion is involving cap. For Coaches in Salary Cap Leagues, squeezing everything we can out of our Cap Space is essential. When we go to the Trade tab, the first few bars could show us our team's current cap position (Salary || Total Bonus Out / Salary Remaining this year). This lets us finagle the numbers by releasing who we need to reach the salary to sign a player in trade or Free Agency.

By the "Accept Trade" button or "Are You Sure You Want to Sign 'X Player'" box, a similar breakdown showcasing what we should have remaining would help, too, so we know what we're dealing with in multi-transaction situations. Simple formulas using existing matrix data is what's used, so it would be a matter of formatting the graphic, the location to display, and dynamic updating mechanics behind the scenes. To make it Truly Ultimate, imagine a Floating Calculator when you hover over a trade/Free Agency pickup with two lines:

This Year's Current Salary + Bonus Out || Salary Remaining
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Salary + Bonus Out AFTER Transaction || Salary Remaining

The Bonus would be the tricky part, since releasing a player with any bonus beyond the current year would result in a salary cap penalty, and if it's substantial enough, it would reject the trade. At least then, we would know WHY the trade didn't go through versus speculating.