Ok so strictly speaking in higher stake cash games, you can get players coming to deals and agreeing to take some money out of the pot, however I've never seen it at games under 5 figure buy-ins, personally. I'm sure they exist, but I've not seen any smaller games do this.
So with the above said I've also never seen a casino allowing lower cash games to take money back, mainly because you're affecting the rake they take. Typically the games that can come to some refund agreements are time games where you're paying for your seat regardless of the pot sizes. In general I would assume that no casino will let you take money out, unless you have asked and a poker room manager has given it the green light.
In terms of the dealer, the dealer should have enforced the bet that was put out and shouldn't have let you call 100$. The bet was 200$ and the dealer should have enforced that.
In terms of this being a valid tactic, not really as the bet was 200$ and should have remained at 200$. Potentially a valid tactic before they bet to try get them to reduce the size. However I will also say a lot of people don't like angle shoots, or soft play so wouldn't recommend it. I think a lot of players will view this as either an angle or soft play.
Also I'd say that's very nice of the player to give the 100$ back, but I feel if players also saw that they'd be thinking of soft play between the two of you. So bear that in mind.