You may have to contact CRA as well as the issuer of this T4A for the final answer on this matter.
You've done the correct thing by reporting the income using the "accrual basis of accounting" as this is when the income was earned. However, the issuer of the slip seems to be using the "cash basis of accounting" and did not issue that T4A until 2018 because this is when you were paid for your services.
CRA may advise that you do not need to report the T4A on your 2018 if you can prove it was earned and claimed in 2017, which would be great. However, I strongly suggest you get clarification from them that this is satisfactory.
Or, you may have to report the T4A on your 2018 return, and adjust 2017 to remove the income claimed, as you certainly do not want to, (nor are you required to), report it twice. If this is the case, it poses a problem if you have applied expenses to this income in 2017 as you will not be able to use these expenses against that income in 2018.
I wish I was able to provide a more definite answer, but CRA will have the decision on how you need to report this.