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If the two lots are in separate corporations or LLCs then you should be showing these entries as intercompany loans rather than sales. One company will have a loan receivable from the other and the other company will have a loan payable on its books. The two loan amounts should be opposites and match in amounts. If the lots are in one entity this also holds true and at year end the two accounts will effectively cancel each other out. There is no proper way to record this activity as sales.
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Floor plan receipts are loans being received and should be recorded as such. It is not income and should be entered as a loan payable. It does not matter which lot is receiving the funds being advanced. The two sides of the entry are debit cash and credit floor plan payable.