Typically you have "constructive receipt" of income when it is made available to you. So "the check is in the mail" is not constructive receipt, but once the check arrives it is. Also, "the check is available at our office in the same town as you whenever you want to come pick it up" is constructive receipt even if you choose not to go pick it up.
Correct me if I'm wrong. It sounds like you made a contract with another person for the sale of land in installments. They've been paying you under that contract, but the contract expired before they completed the contract, so your position is that the land is now yours. But they're continuing the make payments equal to the payments they made before despite there no longer being a current contract stipulating what they should receive in return for these payments.
What's unclear to me, and what I don't have the expertise to answer, is whether this is income at all? I think you've clearly received it if it is income, but if someone sends you a check in payment for something you haven't sold them and you don't cash the check is that still income?
