Was I underpaid?

Had an 8/1 winner that I backed at 2pm. There was a 6/1 shot withdrawn just before the off (4 hours later), hence a 10p r4 deduction.
The bookie is telling me there is a 20p r4 because the withdrawn horse was 7/2 with him when the bet was placed.
Should the r4 be 10p or 20p?