According to a Forbes article dated March 27th, “the stimulus is an advance of a refundable tax credit on your 2020 taxes.” If your 2020 tax return next year indicates that the IRS paid you too much stimulus, they can demand that you return the money they overpaid. If the IRS sent you less than you deserved based on your 2020 return, the IRS will add that amount to your 2020 refund.
That appears to be fair, except that the whole point of the exercise was to put money in people’s pocket now rather than later.
Perhaps, the best way to have achieved that objective would have been for the IRS to send a check to everyone who filed a recent tax return or who is on social security. The IRS can recover any money they may have overpaid certain individuals later (about a year from now) when they review their 2020 tax returns.
It’s better to be roughly right, than precisely wrong.