We had about 4 to 5 gallons of St Arnolds Lawnmower beer left in a 30.5-gallon keg. Will it be OK if I transfer it to one of my 5-gallon kegs before I return their keg? I refuse to send back a keg with beer.
If you have the means to transfer it to one of your own kegs, I see no reason why it won't work. You may have to carb it for a few days though.
There is a very fine line between "hobby" and "mental illness."
As long as it's not exposed to oxygen it should be ok. If your original keg had a regular party type tap that just pumped air into the keg it will be oxidized within a day or two.