457, or 403(b) plan, US Medicare and Federal Insurance Contributions Act (FICA), you must add these contributions to your foreign employment income on line 10400 of your Canadian return. These contributions may be deductible. See line 20700.
https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-return/completing-a-tax-return/personal-income/line-10400-other-employment-income/line-10400-foreign-employment-income.html
Are Your 401(K) Contributions Deductible in Canada?
You might recognize this situation. You are a Canadian resident working in the USA on a TN visa. Your employer offers a 401(K) plan that includes a matching contribution. You know contributing to the 401(K) plan lowers your taxable income in the US. Does the same hold true for your taxable income in Canada?
Provided certain conditions are met, you may deduct, for Canadian tax purposes, the contributions you make to a 401(k) plan in the US.
For example, let’s assume you are a resident of Canada who is employed in the USA and you contribute to your employer-sponsored 401(K). Under the US-Canada tax treaty, your contribution to the plan (up to your remaining RRSP deduction room) will be deductible for Canadian tax purposes. But you need to be careful because your 401(K) deduction on your Canadian return is limited to your RRSP contribution room minus any other RRSP contributions. So if you’ve made RRSP contributions as well as 401(K) contributions, then you may have to defer some of the RRSP contributions you made and deduct them in a future year.
In addition, keep in mind that IRA contributions are NOT treated the same way as 401(K) contributions and are not deductible in Canada.
For example, Canada has a tax treaty with the United States (U.S.). Individuals who are considered residents of both Canada and the U.S. will be considered residents for tax purposes in the country in which they have established the strongest ties. If they have stronger ties to the U.S., they will be deemed non-residents in Canada for tax purposes and their salary and remuneration will not qualify for federal or provincial/territorial film and media tax credits.
https://www.canada.ca/en/revenue-agency/services/tax/international-non-residents/film-media-tax-credits/residency-status-determination.html