I am an independent contractor with my own single-member LLC. I completed a lump-sum work contract in late 2021, invoiced the client at that time, but was not paid until January of 2022. I have a solo 401k with Vanguard, and when I went to the site to contribute part of my paycheck for the 2022 year I noticed the option for a 2021 contribution was still available as I had unused contribution space for that year. I had contributed 100% of my 2021 LLC income into the 401k.
I did some research and found some websites claiming that solo 401k contributions for the prior year can be made until the tax filing deadline (link). So I split my paycheck into both 2021 and 2022 contributions.
It wasn't until later that I considered (1) splitting contribution years from a single paycheck might not be allowed, and (2) contributing anything to the prior year from this paycheck might require that I file that entire paycheck as income in the prior year (undesirable). So here I am to ask:
Will I have to treat that January 2022 income as 2021 income, because I opted to contribute some of it to my solo 401k under the 2021 tax year? Is splitting a paycheck among multiple years in this way allowed? Should I fill out the 401k correction paperwork and send it to Vanguard to withdraw my 2021 401k contribution?
If it's relevant, I currently live in the state of Georgia and my LLC is incorporated there also.