GAPTA--Don't worry about this too much; it's not against any regulation, and the concept itself seems reasonable.
The standard amounts set up by the IRS are done so that there's an easy way for businesses to be able to deduct these reimbursements as expenses on their tax returns. These amounts may or may not bear any resemblence to reality. In the case of the rate for charitable organizations, that rate is set by statute and doesn't get updated regularly the way the business rate does; it also doesn't include some things that the business rate does, so it's apples and oranges. And, given the cost of gas these days, it's probably well below what your volunteer actually spent going to the conference.
Many companies reimburse at a rate higher or lower than the IRS rate, for a variety of different reasons. There is nothing that says that you have to reimburse at a certain rate. But if an organization does choose to use a standard mileage rate, and then they reimburse at a higher rate than the IRS allows, then they just can't take the excess payment as a deduction on their tax return.
So...technically, (if your group files a return) you should make an adjustment on your tax return so that if you paid this guy $300, and by the $0.14 rate you should have only paid $100, you would only list $100 of gas expenses on your tax return, not $300. But, you don't pay any taxes anyway, so, it's really not worth going through all that hassle, especially for a one-time thing.
By paying this guy the higher rate, you reimbursed him not only for gas, but also depreciation/wear and tear on his car, and a portion of a variety of repair expenses, even his insurance. It was probably a lot more than he would have expected, and, for a one time thing, yes, probably more than he "should" have received.
If it were me, for a going forward policy I would just get receipts for gas and base the reimbursement on the gas costs (and any tolls people might have to pay). And come up with a list of circumstances where you would reimburse for gas, so that it doesn't turn into a situation where everyone is asking you for something whenever they drive 2 miles to the grocery store.
Hope this makes sense....