If you can separate the following two points, and live with them. I think you are good to go ahead. Otherwise I would seriously recommend you to reconsider.
#1. Your friend just asked you to give you money so they could buy their dreamhouse.
- Your friend just asked you to give you money so they could buy their dreamhouse.
Are you willing to give out this much money help a friend assuming that you will never get it back? This is what it means to give a gift, don't let their current intentions distract you from this.
#2. Your friend intends to give you some money someday
- Your friend intends to give you some money someday
Will you be happy to wait as long as it takes till he is able and willing to give you some money? Is it ok if this moment never occurs, or would you feel like the money belongs to you already? This is what it means to receive the promise of a gift, don't get distracted by the fact that you may have given them something before.
I don't have a legal background, but if you actually give the money to him so he can buy a house, without demanding something in return, I would judge that you are at least morally ok. (And if the transaction is in cash and fully deniable, you are probably not going to face legal problems in practice).
