PULL the deal. I definitley wouldn't give up more. If someone else offers him a better deal, then more power to him. Don't let this guy use you for leverage!!
This happened to me once so far this year. Guy called me and asked if I wanted Halladay. We negotiated and he said he'd take Cueto and Borowski/ Betancourt for Halladay and 2 throw ins of his choice. I sent the offer, and he rejected it the next day, and sent me an offer also asking for my Adam Dunn for his E Byrnes. I declined, and 2 minutes later he accepted another offer from a 3rd owner!! I was IRATE, and will not deal with this owner again, unless of course I really need/want something he has!!
When I get an offer, I tend to "see what else I can get" for the player in the offer. Nothing wrong with that. I don't however say "I'll do the trade", then announce to others that so and so is availble. That's busch league!!
If I like an offer I will usually say I'll think about it, and then do what I need to do. But verbally agreeing to it, then trying to trade with another owner is BS!! I wouldn't do it. Which is why I almost NEVER verbally agree to a trade.