I don't know much about Sanders' personal finances, but this was a dumb play. Cutting a five-figure check and covering it personally would be PR gold. Mumble something about how it's important that this be figured out, but you don't want your supporters' donations to end up paying for misbehavior, or something.
It's way to easy to spin this in the latter framing, and somebody will. It paints him with the same old out-of-touch good old boy brush, and for his campaign to be in any way viable I think Senator Sanders needs to cultivate a pure-as-the-driven-snow reputation.
(I'm assuming this would be legal as far as campaign finance goes)