McKissic had agreed to a two-year deal with the Bills during the preliminary negotiating period, only to pivot back to Washington when the team let him know it would match the offer. The Commanders had not made him an offer before this period.
There are conflicting reports about what transpired to persuade McKissic to return, and Bills general manager Brandon Beane voiced his frustration last week.
"Once you have an agreement, the agent's supposed to say it's over," Beane said. "And this agent did that. And this agent told the other club it's over. But the other club didn't back off."
Sources told ESPN that Washington was under the impression it would have the chance to match any offer to McKissic.
"So crazy, I don't remember," McKissic said of that period. "My agents called me to let me know what happened.
"You can't ask for a better organization to be chosen by, but I had unfinished business in Washington. The way we left off, we felt we were inclining. ... I had things I wanted to prove in Washington. I made my decision off where I wanted to be. Buffalo is a great organization, but I feel we can do great things here as well."
The Commanders continue to make signings during the free-agent period, reaching a two-year, $8.2 million deal to bring back offensive tackle Cornelius Lucas, agent Dan Saffron told ESPN on Thursday.