I don't think you should try to translate it any more closely than this. You just need a title that explains what the contract is for.

Private Deed also works in the USA, as refers to a dealing in land.

... building which will / to be built = off-plan is slightly ambiguous as could mean on the same land or a different plot / tenement, possibly sending the translation a different equity-sharing way.

barter: exchange (goods or services) for other goods or services without using money.
"he often bartered a meal for drawings" (Lexico.com)

real estate barter: https://www.google.com/search?q="real estate barter"

Something to be aware of:

Promise
While every contract contains some form of a promise, it is often difficult to say that a promise is the same as a contract. A promise however may be the first step to in entering into a contract.
A promise can be described as an offer which is an important element in forming a contract but a promise alone may not be binding on both parties. This is so because an offer alone does not oblige the parties to respect any terms. Furthermore, a promise can lapse with the passage of time. Or it can also be rejected by the one to whom the promise is made. It is in this sense only a commitment by one party to another to carry out or refrain from carrying a specified act or acts. A promise can be binding or not but difficult to enforceable in law. https://www.virtualkollage.com/2018/03/The-differences-betwe...

Sugestão.