It's pretty easy to work out really.
Di Maria was bought for £59.7m (ignoring other costs like signing on bonuses, agents fees etc) on a 5 year deal. From an accounting perspective that's £59.7m spread out over the length of his 5 year contract, so a £11.94m cost per season. He's been at United for 1 season, so £11.94m of his cost has now been written off, meaning Di Maria's "cost" to United for the remaining 4 years will be £47.76m. If they sell him, tomorrow, for the reported £44.5m, that means, from an accounting perspective United will lose £47.76m this season (all his remaining "cost") but will add £44.5m (his sale value). £44.5m - £47.76m = -£3.26m. So, United will actually record a net loss of £3.26m on Di Maria.