Is a contract valid if a seller stipulates retaining use (usufruct) of the sold item for a specific period?

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The contract is invalid if one sells a dwelling and simultaneously stipulates renting it out for a month. This is because the sale grants the buyer immediate ownership of the usufruct, and stipulating a payment in return for what the buyer already owns by right of purchase invalidates the transaction. This ruling is analogous to the Prophet's prohibition of 'Qafeez al-Tahhan' (a specific measure of ground grain), which implies one should not contract for a miller's service in exchange for a portion of the output, as it makes the labor a compensation for part of the service itself.