No, a team cannot be involved in a retained transaction for a player that was on their roster within the last year if the player is still under the same contract.  So in your example, if Calgary cannot re-acquire Monahan until June 30 in a retained salary transaction.  They could re-acquire him next Fall in a retained transaction because he's a free agent this summer and he would be on a new contract.  Similarly, Montreal cannot trade Monahan and retain salary and then re-acquire him back within a year (or until he's on a new contract).