The Portuguese moved to Los Blancos from United in 2009 and has scored 256 goals in 251 games for the Spanish outfit, including 17 in last season's Champions League title-winning campaign.
It is understood that the attacker has become disillusioned with life in Madrid, however, with various reports suggesting that a return to Old Trafford next summer could be on the cards.
Balague, who broke United's move for Angel di Maria this summer, has said that there is a willingness from both parties to make the transfer happen.
"I do believe Manchester United are preparing an offer, or have got an offer prepared, for Cristiano Ronaldo and have had for months," he told Sky Sports News.
"I do believe he is thinking of going back, but there will undoubtedly be more on that later!"
Ronaldo scored 118 times in 292 games for United between 2003 and 2009.