It is understood one reason why negotiations became so protracted was because Johnson's first club, Wycombe Wanderers, are entitled to a 15 per cent sell-on clause.
"We spoke to Roger on Tuesday and he made it clear that he wanted to leave but only on the right terms, which he will do because he is a model professional," said Ridsdale.
"Every player has his price and we have achieved an acceptable price. It's only fair on Roger that we allow him the opportunity to play in the Premier League.
''We wanted £5 million and that's what we got, it will show good value for a player we invested £275,000 in."