I can't see this happening, but perhaps a system like the following could work:
1) 2 managers agree about the players they want to exchange
2) they have to list all the players who are part of the trade on the TL within say 4 hours from each other
3) these players will have a mark of some kind to let people know their managers are trying to trade them. People will bid normally on all players (and they will buy them if the trade fails)
4) the managers in the trade cannot bid on the other manager's players with any of their teams
5) when all the auctions end, the result of the bidding needs to be screened by someone to make sure nobody submitted some significant raise out of the blue, to "prop up" the value of one of the traded players to make the values match
6) the manager who is short on values from his side, will have to automatically pay the difference in cash. If he can pay the trade automatically happens, if he can't we move to point 7)
7) 24 hours after the last bid the trade resolves: if one side fails to have bids on all the players he listed for the trade OR if one side can't gather enough cash to push the trade through OR if the review at point 5), which will have to be done in 24 hours, highlights some strange bid, then the trade fails and there is no exchange. In the last case, the suspicious bids will be reported to GMs and dealt with all the usual penalties.
8) if the trade fails, the managers can outbid the TL winning bid on their own players by 2% for a limited amount of time (say 30 minutes after the trade is resolved and failed). If they do not or they don't have the cash, then the winning bids from the auction 24h earlier will receive the players.