It's a little unusual, but I don't think the financial terms are completely unreasonable on their face.

What you describe is similar to an *interest-only loan*, where you make payments that only cover the interest due each month, and the entire principal is due as a single "balloon payment" on a specified date (in this case, the date on which the condo is sold).  Your monthly payment of $500 on a principal of $115K is equivalent to an annual interest rate of 5.22%, which at least is not completely usurious.  With a traditional mortgage you might pay a rate as low as 3%, if you had excellent credit - but I don't know, from what you've said, whether that's the case.  Did you make the current arrangement because you were unable to get a loan from a bank?

The main difference here is that instead of the balloon payment being a fixed $115K, it's "75% of the gross proceeds of the sale".  If the condo eventually sells for $155K, that would be $116,250, so that's slightly advantageous to them (assuming that "gross proceeds" means "before deducting commissions for either the buyers' or sellers' realtors or any other costs of the sale"), and thus slightly disadvantageous to you.  If the condo appreciates in value, that's more of a win for them and more of a relative loss for you.  But it's also possible that the value of the condo goes down, in which case this arrangement is better for you than a fixed balloon payment.  So this deal does prevent you from getting a larger share of any gains in the value of the property, but it also helps insulate you from any losses.  That's important to keep in mind.

There's also the issue of needing their consent to sell.  That's potentially problematic - usually in a joint ownership scheme, *either* owner has the right to demand to be bought out or to force a sale.  I guess it depends on whether you think your parents would be likely to consent under reasonable circumstances, or to insist on holding the property against your best interests.

It's true that you aren't building equity with this arrangement, and if you thought you were, you are mistaken or misled.  But let's compare it with other options.

If you would qualify for a traditional 30-year fixed mortgage at 3%, your monthly payment would be slightly lower ($484), and you would be building some equity because your payments would reduce the principal as well as paying the interest.  But a 30-year loan builds equity very slowly at first - after 7 years you'd have only about $20,000 in principal paid down.

If we assume that 5.2% represents the interest rate you'd otherwise pay based on your creditworthiness, then your monthly payment would be $631.  So compared to that, you have an extra $130 per month that you can save or invest in whatever you want - you're not forced to invest it in your house.

Note that in either case you'd still be paying the condo fees, property taxes, insurance, and maintenance yourself.  So we might as well eliminate those from consideration.  

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It might be a good idea to find out what other options you would have - perhaps try to get an interest rate quote on a traditional mortgage from a bank, based on your credit.  Then you can decide what to do, taking into account:

- your financial situation; how much of a monthly payment could you afford?

- your relationship with your parents; are they likely to be reasonable about renegotiating?  Do they in general tend to respect your wishes?  Would it harm your relationship if you tried to get out of the deal, and how important is that to you?

- To what extent do you actually *want* to pay for equity in this property? Do you really believe it's a good investment, and have evidence to support that?  

Your options include:

- Try to renegotiate the terms of the loan from your parents

- Try to "refinance" the loan, by getting a loan from a bank and paying off some agreed-upon amount of principal to your parents

- Try to force the sale of the condo and move to another house, financing it some other way

- Consult a lawyer as to whether your agreement with your parents is legally enforceable.  For instance, do they have a lien on the property?