Dombrowski had to increase his offer by 1 year and $32 million. He had nothing to gain by letting it out. The Cubs didn't have his name on the dotted line, they had nothing to gain.
Dombrowski also had nothing to lose by letting it out.
He wanted to sign Sanchez. To sign him he had to increase his offer, and apparently he decided to increase it enough that he thought the Cubs would not come back and top it.
Whether the Cubs' offer to Sanchez was reported or not, Dombrowski would have had to increase the offer to sign Sanchez.
Even with Theo, the fact that the Cubs' offer was reported before the signing was final changed nothing in the negotiations with Sanchez. The only possible effect was to let Garza know that the Cubs were willing to go that high, but if it had NOT been reported, Garza would still have seen what Sanchez and other comparable pitchers were getting, so it likely would not be a meaningful factor in Garza's demands if the Cubs try to extend him.
Did the agent leak it?
Perhaps, but the idea that everyone else would have somehow been hurting themselves by leaking, and that therefor the agent MUST have been the source, really doesn't hold up to scrutiny.