You’re offered $1000 USD for every 1 lb (0.45kg) you'll instantly gain. The weight is added to your body right now, with no prep time to lose weight in advance. You get the money immediately, but it’s up to you to deal with the health consequences or lose the weight afterward (if you choose to). It's a one-time deal and can't be reversed.
For example, if you choose to gain 1000 lbs (so you gain $1M), it could be fatal. The weight will be evenly distributed. Your body will auto-adapt to the weight as if it had gained that weight over time naturally but even still, too large of a weight will still prove to be fatal or have serious health issues.
How much weight, if any, would you take for the cash?