Have you informed the slip provider that the T4A is incorrect according to your records?

I only just discovered this, and I have to submit my taxes before I can sort this out.  My plan is to submit the incorrect (higher) amount shown on the T4A at the moment and adjust it later.

Submit using the higher amount(between the T4A and the actual amount) and yes, once you have received the amended slip, file an adjustment.

https://turbotax.community.intuit.ca/replies/2647992