Your correction is close to the mark, at least to me. Cash's current value is related to its future value. In finance, it's termed present value. Present value is generally a discounted value of future cash flows. The present value of an annuity (a stream of payments over a defined time period) is less than the sum of the payments themselves. The discount represents the risk, usually inflation, against the future values.
When cash is involved in a trade, the risk is that you get it, but cannot use it. For the one sending it in the trade, the salary is covered because the trader may not have a need for it (thus reducing their own risk of losing unused cash at the end of the season) and covering the cash of a traded salary brings back better value. For the one receiving the cash, it can either be used immediately for another purpose, or it may help to afford the player's salary, or be held (with risk of non-use) for some later purpose.
In all of those scenarios, it is a valuable working part of exchanging value. It has value whether or not it is converted. It's value is determined already within the trading and market context.