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HMRC internal manual

Employment Income Manual

The general rule for employees expenses: relief is limited to expenses actually incurred: example

An employee has to travel on business. Her employer pays her a travel allowance that isenough to buy an airline ticket. She actually buys a rail ticket at less cost. She candeduct the expenditure she actually incurred, the cost of the rail ticket. She is taxableon the payment from the employer less the amount she actually spent.