No, you are deliberately misunderstanding the difference between gross and net to score a political point.
We owe then £350m gross. That is mandatory.
They pay us £100m back. That is discretionary.
I want to vote against that discretion. So performing a simple subtraction is MISSING THE POINT.
The rebate/discount is not discretionary. Also, the discount is not paid back because we don't pay it in the first place. On QT one Brexit was arguing that we have no control over how we spent the discount but she was corrected by Farage. I've heard some odd comparisons, the usual being that when you tell someone your salary you tell them the gross figure. Well the difference is that all the deductions are things you are due to pay, your tax, your national insurance, your pension. When you give your salary, you don't include employers national insurance contributions or any pension contributions made by your company.
https://fullfact.org/europe/our-eu-membership-fee-55-million/