Refusal of a full refund

I went to buy my daughter a sewing machine as a birthday gift and Elna's did not have machine in the shop and told me they sold the last machine in the morning and I decided to pay for it as I had money with me. Later in the week I went to ask for a refund as my daughter said she does not want machine and they are telling me that they are going to deduct 15% and the machine had not left the shop and the receipt does not stipulate that term and condition, therefore I need my full my refunded