On your T4, the purpose of Box 20 is for Registered Pension Plan (RPP) contributions to be deducted, since you specifically stated that the contribution was a Group RRSP, it indeed should have been reported on Box 14 as part of earnings. However making changes yourself may pose a risk for inaccuracy; I would suggest you contact the employer and ask why it was done this way. If it was indeed a mistake then it would be safe for changes. The change would then be:
-> A subtraction from Box 20, and an addition of the same amount into Box 14, as well as a RRSP Contribution to be reported.
For more information, check out: TurboTax - T4 Slip Information
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