What is the ruling on my swearing by Allah not to take money from my friend, then taking it after his insistence?

You have broken your oath by taking the money you swore not to take. If your intention was to honor your companion without obligating him, then the preponderant opinion is that no expiation is necessary, because the intention was honor, and that has occurred. However, if your intention was to obligate him to take the money, then an expiation for breaking an oath is incumbent upon you.