修正後

Withdrawal.go

package usecase
import "strconv"

type Withdrawal struct{
  mOutputBoundary OutputBoundary
  mDataAccessor DataAccessor
}

func CreateWithdrawal(aOutputBoundary OutputBoundary,
                      aDataAccessor DataAccessor) *Withdrawal{
  return &Withdrawal{
    mOutputBoundary: aOutputBoundary,
    mDataAccessor: aDataAccessor}
}

func (r Withdrawal) Input(aDataSource map[string]string){
  aDataSource["process"] = "Withdrawal";

  amountOfMoney, err := strconv.Atoi(aDataSource["amount_of_money"])
  if err != nil || amountOfMoney < 0 {
    aDataSource["result"] = "Failed";
    r.mOutputBoundary.Output(aDataSource)
    return;
  }

  balance := r.mDataAccessor.GetBalance();
  subtratedBalance := balance.Subtract(amountOfMoney)

  aDataSource["balance"] = subtratedBalance.ValueAsString()
  aDataSource["result"] = "Success";

  r.mDataAccessor.UpdateBalance(subtratedBalance.Value())
  r.mOutputBoundary.Output(aDataSource)
}