Help! Today I placed a �10 each way bet on a horse that won...?

when I went to collect my winnings the guy turned round and said that I only left �10 on the counter instead of �20 (even thogh I'm sure I left a �10 note and ten quid in change... so anyway he said he changed my betting slip to �5 EW. I only live over the road and he knows me. Anyway my question is, if he accepted the �10 EW bet is he llowed to change it? I've beenback and he says I'll have to see the owner tomorrow morning, I'm not sure what the law is on this one...