Two diverging strokes at top — the position-marker form of 八 'split,' placed above to indicate a parting upward. Here it signals breath issuing from the mouth beneath. Indexed under Kangxi #10 儿 (ér) by tradition, drawn from the legs in 兄 below. Same marker caps 公, 分, 父.
Bottom 兄 — 口 mouth atop 儿 person, originally an officiant pronouncing prayers. With the splitting-breath marker above, the picture is a person speaking outward to exchange information — the sense of 兑 'to exchange,' later 'to redeem cash.' 兑 is also a 八卦 trigram representing 'lake.' Indexing radical: 儿 (Kangxi #10).
Filed under radical 儿 (ér) by convention. 儿 is not a separate component in 兑, so no strokes are highlighted.
