United completed the signing of Robin van Persie on Friday, but Ferguson has indicated that his transfer business is not yet complete.
"I think that is maybe it [in terms of signings this summer]. Maybe in one other position," Ferguson told MUTV.
"We have spent a big amount of money and, if I need to go to the owners, I am sure they would say 'yes'. But it's only if a certain player would become available."
United face Everton in the Premier League on Monday.