A contract to sell a house in the future can be legal and enforceable, but whether this specific one is legal or enforceable depends on its exact wording. There are at least two possible issues with the contract you describe, however:1) If it states that you will "take over the mortgage", then since you cannot do so unless the bank voluntarily agrees to let you "assume" the mortgage, the contract may be void for impossibility (void because it is an agreement to do something which is legally impossible--to take over a mortgage without bank approval) and/or be terminated for breach if you try to take over the mortgage but cannot. Simply *paying* her mortgage is something you can do, but since you cannot necessarily "take over" the mortgage and put  it into your own name, if the contract requires that, the contract may be fatally flawed. Promising to do what is impossible voids a contract; promissing to do something but then failing to breaches it.2) If there is no date by when you MUST conclude the transaction, then the "contract" lacks a critical term and does not in fact form an enforceable contract. To be enforceable, all "material" terms, which includes when the contract will be performed (and the exact sale price) must be defined. You write that under the agreement, you will purchase the home "once a real estate asset of mine is either sold or rented out." That is an open-ended, undefined time: it could be tomorrow, it could be in 6 months, or a 1 year, or 5 years, or never. The law will not obligate her forever on a contract where there is no guaranty of when or whether it will be fulfilled. So if there is no set end date--i.e. the contract does not require you to buy the home by no later than a certain defined date--the "contract" is likely not actually an enforceable contract, since a material term is undefined.So in theory, your contract could be enforceble, but depending on exactly what it says, it may have fatal flaws. Bring a copy of the contract to a real estate attorey to review it's specific language; the lawyer can then advise you as to its enforceability and your rights.