If someone is willing to do this, they are likely to be people who currently are on low incomes and thus unlikely to be paying much tax if any. I think it would

If someone is willing to do this, they are likely to be people who currently are on low incomes and thus unlikely to be paying much tax if any. I think it would have the opposite effect though. In most countries, the welfare system harshly punishes people who do a small amount of work, withdrawing benefit at a very high marginal rate. This is a massive disincentive to finding work. With a basic income, people can for example do voluntary work. People on sickness benefit can gradually try to go back into work, doing a few hours per week. If you can top up your income without being penalised for it I reckon quite a lot of people currently on welfare would start working a little.