I presume you have his money. Don't send it back over this issue. Your gun's chokes are well within the range known for this model. The buyer should be liable for return shipping and I'd for sure collect that money prior to shipping.
Most likely, the buyer and 'smith (so called) didn't know that choke constriction, measured in thousandths of an inch ("11" = 0.011 inch constriction in diameter from choke entry to exit(eleven one thousandths of an inch)) gets smaller for smaller gauges. Full for 12 gauge = 0.035" where 20 gauge full = 0.025".
DDA
