They never just send over a draft pick or player anymore to complete a previous deal, like we used to see. All transactions do need to be completed at the time of the trade call.
However, when you see "Team A trades their 7th rd pick in this year's draft for Team B's 7th rd pick in next year's draft" on Draft Day, those types of trades are very much in the vein of "Hey, remember when I did you that favour previously and you owe me future considerations? Well, take this into consideration.", but we'd never actually know it was related to the previous deal officially or not.
Essentially, f.c. deals build relationships and garner some reciprocity for GMs to "consider" in the future.
Pretty much the only f.c.'s that you can point to with confidence are when Team A trades an NHL contract playing in the AHL to Team B for future considerations, and then at the same time Team B's AHL team sends an AHL contract back to Team A's farm team. Even then, in the eyes of the NHL these are technically unrelated transactions because the NHL has no care or say in AHL contracts, but it's pretty heavily implied that the AHL transaction is the "consideration" taking place to move the NHL contract.