Boss Martin Ling told BBC Radio Cambridgeshire: "We've had an offer from a Blue Square Premier club for Robbie that didn't meet the valuation.
"There is a valuation that the board and I would have, whether they're the same at the moment is another matter."
The U's are actively
looking for new investors
but Ling says there is no "fire sale" at the club and he is willing to hold out for an offer which matches his valuation.
I believe I've made him a better player, I do think he can play in the bottom two leagues of the Football League
Cambridge United boss Martin Ling
And former Swindon winger Ling is confident academy product Willmott, whose contract expires in June, is happy with the situation.
"Robbie's fine with it, he understands fully," he said.
"Robbie fully accepts that I probably get the best out of him, more than anyone else.
"That's what he tells me, whether he tells other people different I don't know.
"I believe I've made him a better player, I do think he can play in the bottom two leagues of the Football League, I'm not too sure yet if he could do the top two leagues.
"I think he needs a stepping stone if the opportunity comes along."
Source: BBC Sport