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In such instances, because the agreement is month-to-month, the landlord is entitled to charge for the entire second month. The only way around this is if the landlord agreed to the 15 days and agreed to half the payment.
First, it is illegal for him to take the equipment to pay for the rent. You can sue him to recover the property or possibly press charges against him for theft (however, the police might say that this is a civil matter and not a criminal matter and therefore they won't get involved). Additionally, he would need to sue you to recover the other 15 days. You would argue that he verbally agreed to the modification, but the issue is that you don't really have much evidence unless he admits to it. In that case, the written contract would probably prevail. But if you give him payment for half of the month, he might not sue because of the potential headache.
If the police said it was civil, then there isn't any other crime he is committing by holding it. I hope this has been helpful. Please let me know if my answers have been satisfactory.