Mixing tenses?

His winnings were typically no more than a couple dollars, but that morning after scratching off the third golden coin, he was able to cash in for $1000.

I'm sorry, I'm not a native speaker. The sentence above sounds correct to me, but I'm not sure whether it is grammatically ok to write "after scratching off the third golden coin" between the past tense of the sentence. If anyone could help, I would be very grateful.