The midfielder could face his former team-mates tomorrow at The Den after the two clubs agreed to the unusual clause in the deal – an initial loan that will become a permanent transfer in January – that took the 26-year-old to London last month.
Loan moves usually prohibit the player from playing against his parent club.
Leonard only arrived at Bramall Lane in a £750,000 switch from Southend United in January, but struggled to break into Wilder’s starting XI.
So the Blades sanctioned a £1.4m move to Millwall last month, with the likes of Ollie Norwood, Marvin Johnson and Martin Cranie arriving to bolster United’s squad.
On the possibility of facing Leonard tomorrow, Wilder explained: “It was part of the agreement, it’s a permanent switch, Ryan is not coming back. It’s not like he is our player and is coming back to us. In all but name it’s a permanent transfer.
“It was a situation which we felt was right for Ryan – he was desperate to play games.
“It suited the player and we got a really good return on the fee which we paid. We invested it elsewhere to make us stronger.”
Leonard made 13 appearances for United last season, but started just once this campaign, in the EFL Cup tie against Hull City. His other three league outings this season came as a substitute.
“He was a good kid, Ryan, and a really good professional,” said Wilder. “He came to me about 10 days before (the move) and asked what he had to do to get in the team. I said, ‘keep doing what you’re doing’. There was no major plan to move him on. Millwall lost a player for decent money, they asked about Ryan and we asked about someone else to make it happen. Ryan is playing for them and we’ve done some bits to strengthen us.”
Millwall sit in the bottom three – with just one win from their opening nine games – but Wilder will not underestimate the hosts.
“I never look too deeply into starts. Millwall is a tough place to go, we understand that,” he said.
“We will come up against a real committed side. They beat Derby, they should have beaten Middlesbrough, and they could easily be a lot higher up in the division.”