In the US, employees are sometimes protected by the WARN Act which requires companies to pre-notify employees before eliminating their positions. The pre-notification period might be 2 or 3 months and it might be casually referred to as severance even though it is not. At the end of this period, the employee is terminated and may receive actual severance.
In practice a lot of companies tell their employees to go home and not come in during that time. The company will deactivate their badges, email, and network, but the employee is still employed – they get paid normally, continue to get benefits, they might still qualify for bonuses.
So a long answer to your question – the portion triggered by the WARN Act will be paid out over time and there wouldn’t be a way to get a lump sum. The severance is probably a lump sum.
To answer the OP, I had a situation similar to this happen to me. I worked at a remote office that was consistently downsizing. At one point they decided to close the office; they offered severance and some people got retention incentives. We were expected to continue working for the pre-notification period. A few weeks later they changed their mind and kept the office open. The severance was rescinded, but the retention bonuses could not be. I decided it was time to find a new job and not wait around for a bonus. It felt good to take control of the situation rather than be bounced around by the whims of the decision makers.