會計~tade-in of fixed assets

Trade-in Allowance simply means the deemed price the seller would buy back your old asset i.e. the proceeds on your old asset 'sold' to the seller of the new asset.
In your case, you have to state out the gross price in your fixed assets account, e.g. Oringinal Cost $200, Trade-in Allowance $ 60 the money you have to pay to the supplier is $200-$60 = $140
So, you have to Dr. Asset $200 Cr. Cash/Bank $140 Cr. Asset Disposal ( Trade-in Allowance ) $60. This Asset Disposal account will contain the cost of the old asset and its accumulated depreciation and the reduction of this 'proceeds( Trade-in Allowance )' will be reflected in the Profit and Loss Account as Profit / Loss on disposal of asset.