Hi Alain,
I agree with Dave & Deb going outside of Airbnb is too risky. In fact, if they don't have the money to pay now, there is no guarantee they will have the money later.
What does your gut feeling tell you about this guest? Do they have several reviews positive, negative or neutral?
You may want to call Airbnb and explain your guest's predicament to see if they can look into his/her account and see if they have a good standing history. If the person is new, I would be more worried. IF the guest actually has a good standing history (per Airbnb) you may want to offer the guest that you will keep them abreast of potential inquiries.
You certaintely wouldn't want to block your calendar as you may lose out on receiving inquiries or requests to book from other financially capable guests.
Their payment through Airbnb is done through a credit card so if they don't have the credit available, that is another red flag. You don't want to end up with a squatter. As much as we may want to help a person in need, as hosts we have to be cautious of potential scammers.
Keep us posted.