Maegan, I won’t make any judgment on whether you’re nuts or not (instinct of self-preservation and all that), but here’s how the whole transfer of money and products in this riddle goes:
1. At the start, the buyer has the fake $20, the seller has the $10 hat, and the neighbour has $20 dollars:
B = $20 (fake)
S = $10 (the hat)
N = $20
2. The buyer gives the fake $20 to the seller:
B = $0
S = $30 (the hat and the fake $20)
N = $20
3. The seller gives the fake $20 to the neighbour:
B = $0
S = $10 (the hat)
N = $40 (including the fake $20)
4. The neighbour gives $20 back to the seller:
B = $0
S = $30 (including the hat)
N = $20 (fake)
5. The seller then gives the hat and $10 to the buyer:
B = $20 (including the hat)
S = $10
N = $20 (fake)
6. The seller then has to give $20 to the neighbour:
B = $20 (including the hat)
S = $-10
N = $40 (including the fake $20)

