You also lose the asset associated with the player. For example, Garnacho was bought for £40M, amortised over 5 years. So his current book value after a year is £32M. If you sell him, you lose a £32M asset. So whatever you sell him for, you take away £32M from that number in terms of accounting profit and loss. If you sell him for £30M, then you'd net a £2m loss, making things worse. If you sell him for £40M, you gain £8M profit.
There is salary to account for, but that's a much smaller number than the transfer fee, maybe a £2m saving a year. Worth doing, but if you're trying to make up losing £80M in CL income, there's a lot more to go.