They aren't giving up Paul George, but I'd make the trade for the trade exception and a first (I'd try to get two firsts).
If we needed a player back, I think George Hill might be available. If Granger keeps up his less than stellar play, they might be willing to move him as well, though I doubt it in a trade for Allen as that really is a salary dump type trade and I'm not sure Indiana is at that point. There are some other trades they might consider centered around Ray.
These trades I'd probably do if I'm the Celtics (in order that Indiana is most likely to do)
Allen for 1st (or two)
Allen for Stephenson, Admundson
Allen for Hill (would add Bradley if they wanted him)
Allen for Granger (not very likely)
I'd take on Foster if it was a requirement from Indiana, but wouldn't want to waste most of the trade exception on him so I would push back on that requirement.