For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Is the funding being given by the friend for the purpose of buying something on the friend's behalf (that is, running an errand for the friend in order to bring whatever is bought back and give it to the friend), or is it being given by the friend for the questioner to buy something that the questioner himself needs or wants at his own discretion?
Either way, the only ways that I can see that sin would be involved in such a situation would be if the person who was given those resources somehow used them in a manner or for a purpose that the friend did not intend, or did not return the full amount of any unspent resources to the friend against the friend's wishes or instructions, or used the credit card in an unauthorized or illegal manner (either in regard to the amount spent, the items purchased with that amount, or using the credit card while not being an authorized user of it).