I think the date he was deported was 1938. The date on the front is less clear, but it would make sense it was the same date he arrived. I don't have access to this database, but if you go back to this entry there, the cards were hopefully filmed in chronological order. Take a look at the cards before or after to see if the handwriting is more clear, so you can see what year they were all arriving.
I took the act of 1924 to mean that was the law in effect at the time he arrived, so the reason he was turned back would be in those rules. In addition to the Asian exclusion, there were quotas for all the countries. It's possible we had reached the quota for immigrants from Canada by that time. In 1924 it would have been all immigrants from Great Britain, so it would have included people from other countries besides just Canada.
The $25.00 is money Shown, not paid. I know they had to have a certain sum of $ in order not to be considered a person who would likely become a pauper. This amount was raised over the years as well, but I don't know what it was in 1938.
There could also have been some problems with the dates he reported being here. On the front of the card, it was originally 10/3/1916, then that was crossed out and the date Spring of 1923 was filled in. Maybe they thought he was lying to them?