The rebate is set out in the EU’s Own Resources Decision. This does not (as some suggest) have an expiry date (other Member States’ rebates will expire in 2020, but the UK rebate, and the law as a whole, will not). If the EU wants to amend this law, Article 311 TFEU applies:

The Council, acting in accordance with a special legislative procedure, shall unanimously and after consulting the European Parliament adopt a decision laying down the provisions relating to the system of own resources of the Union. In this context it may establish new categories of own resources or abolish an existing category. That decision shall not enter into force until it is approved by the Member States in accordance with their respective constitutional requirements.

It’s clear that the UK government can control the future of the rebate by means of a veto. Furthermore, so can Parliament, since the ‘constitutional requirements’ for the UK referred to mean that an Act of Parliament has to be passed for any amendment to the Own Resources Decision. These constraints have meant that the veto has stayed in place for over 30 years – although the UK government and parliament have agreed to some reduction in it over that time.

Source: EU Law Analysis: EU Referendum Briefing 1: Can the UK control the EU’s future if it stays a member?