'...our “rebate” is deducted before the net amount is sent.'
This is an oft repeated fallacy. The rebate can only be calculated retrospectively because of its complexity and the initial provisional calculation is monitored over four years as 'better' revenue data emerges.
To quote the European Parliament:
"The initial calculation of the UK rebate for a given year is budgeted and paid the following year, on the basis of the provisional values of the variables that play a role in the calculation and financing of the mechanism. The definitive amount of the rebate is known (and any difference with the provisional amount is accordingly budgeted and paid) four years after the year in question, on the basis of the final values of the relevant variables."
"For example, this means that the 2007 UK rebate was budgeted in the 2008 EU budget in terms of a provisional amount, while the definitive amount for 2007 was calculated and budgeted only in 2011. In 2011, an amending budget addressed the difference between the provisional and definitive amounts of the 2007 UK rebate. In addition, between 2008 and 2011, the provisional amount was regularly monitored in order to budget any significant change in the estimate of the rebate."
http://www.europarl.europa.eu/RegData/etudes/BRIE/2016/577973/EPRS_BRI(2016)577973_EN.pdf